the leader of the Party, the political collective that presides overOrwell’s fictional society, Oceania, and he is the face of this autocratic system. As 1984 begins, the novel’s protagonist, Winston Smith, is described as ascending the staircase of his apartment building, the elevator not being an option due to yet another electricity shortage courtesy of the inefficiency of the totalitarian regime about which the reader is to learn quite a bit:

“On each landing, opposite the lift-shaft, the poster with the enormous face gazed from the wall. It was one of those pictures which are so contrived that the eyes follow you about when you move. BIG BROTHER IS WATCHING YOU, the caption beneath it ran.”

Big Brother is a prominent if not always visible presence throughout Orwell’s story. Modeled after Joseph Stalin, the indescribably brutal dictator of the Soviet Union, even described by the narrator as possessed of a thick black mustache, similar to that worn by Stalin, Big Brother is the face of the regime. The face of the regime’s opposition is that of Emmanuel Goldstein, a former leader of the Party alongside Big Brother who has rebelled against the regime he helped to create. Hate Week, an annual ritual designed to rile the masses against the nonconformity embodied in Goldstein, will provide the spark that ignites Winston’s own decision to rebel against Big Brother and the regime he rules. In a key passage, Orwell describes this transformation as follows:

“In a lucid moment Winston found that he was shouting with the others and kicking his heel violently against the rung of his chair. The horrible thing about the Two Minutes Hate was not that one was obliged to act a part, but, on the contrary, that it was impossible to avoid joining in. Within thirty seconds any pretence was always unnecessary. A hideous ecstasy of fear and vindictiveness, a desire to kill, to torture, to smash faces in with a sledge-hammer, seemed to flow through the whole group of people like an electric current, turning one even against one’s will into a grimacing, screaming lunatic. And yet the rage that one felt was an abstract, undirected emotion which could be switched from one object to another like the flame of a blowlamp. Thus, at one moment Winston’s hatred was not turned against Goldstein at all, but, on the contrary, against Big Brother, the Party, and the Thought Police; and at such moments his heart went out to the lonely, derided heretic on the screen, sole guardian of truth and sanity in a world of lies.”

This lengthy passage is included to emphasize the centrality of Big Brother to Orwell’s fictional society. The Party that Big Brother leads, and that governs this dysfunctional society, uses propaganda and disinformation to keep the masses loyal and compliant, although the ubiquitous threat of torture always looms in the background in the event the more passive forms of control prove inadequate. The importance of Big Brother to 1984, then, is the centrality of this threatening and all-seeing presence to the autocratic society Orwell depicts. He is what the Communist Party of the Soviet Union once called "First Among Equals" in an Orwellian attempt at explaining the emergence of a single dictator in a system in which class distinction are supposed to have disappeared and the Party exists solely to advance the interests of the masses it, in reality, invariably exploited and enslaved.

Big Brother is a father figure, who, like Stalin or Hitler, is supposed to be deeply beloved, honored, and obeyed by all the people in his country. He is a symbol of the state and the Party. The Party wishes for him to become the only being anyone loves and to be followed with slavish, mindless devotion.

Big Brother is a threatening father figure, too, which the motto most associated with him suggests: "Big Brother is watching YOU." This motto could mean that he is watching over and protecting each of his people, as a good leader might, but in reality, it means that the Party has its citizens under constant surveillance. Any deviation from orthodoxy is severely punished.

One of the chief signs of Winston's deviance, in the eyes of the Party, is his hatred of and desire to destroy Big Brother. However, by the end of the novel, he has adopted the proper slavish attitude of complete devotion to this figure.

Big Brother is the physical representation of the oppressive government of Oceana.  It's not clear whether or not Big Brother is an actual person, but I do not believe that he is.  There are posters of Big Brother all over the place, but I believe that he is a representation of the government (as your question implies).  "Big Brother is Watching You" is what is written on the posters, and there is no way one guy could watch everybody.  But a government body could.  Orwell does a nice job of explaining in chapter 1 how a government could do that.  

"The telescreen received and transmitted simultaneously. Any sound that Winston made, above the level of a very low whisper, would be picked up by it, moreover, so long as he remained within the field of vision which the metal plaque commanded, he could be seen as well as heard. There was of course no way of knowing whether you were being watched at any given moment. How often, or on what system, the Thought Police plugged in on any individual wire was guesswork. It was even conceivable that they watched everybody all the time. But at any rate they could plug in your wire whenever they wanted to. You had to live — did live, from habit that became instinct — in the assumption that every sound you made was overheard, and, except in darkness, every movement scrutinized."

In a nutshell, Big Brother represents government control and oppression.  It watches everything citizens do.  It controls their daily schedule.  It controls their thoughts.  It attempts to control their emotions.  It even controls the past.  

’In memory. Very well, then. We, the Party, control all records, and we control all memories. Then we control the past, do we not?’

I do not believe its meanings changed over the course of the novel.  I believe its powerful grasp was simply driven further into the reader's mind and Winston's mind. 

Big Brother is not really a character; it is a metaphor for the power that the Party has and for all dictators and despots the world has ever known.  The Party in 1984 uses Big Brother to intimidate, to rule, and to inspire loyalty.  Posters of Big Brother are everywhere in Oceania.  His face is more familiar than any other.  The people are told that Big Brother loves them and takes care of them, so that most people love and are fiercely loyal to "him," and thus to the Party.  Decisions are made by the Party in the name of Big Brother and because the people love him, the Party is free to establish any laws they want and they have total rule over the land.  The many posters tell people that Big Brother is watching them, which in many ways he is.  The Party uses two-way telescreens and hidden microphones and cameras to keep watch on people.  Because they know they are being watched, people are less likely to do anything they know would displease Big Brother.  Also, the people are less likely to defy Big Brother because they fear the retribution.