The overall message is that totalitarian governments such as those of Nazi Germany and Soviet Russia are/were bad.
When Orwell wrote 1984, he was concerned that governments were moving more toward totalitarianism. He worried that these governments might start taking away more and more of people's rights and freedoms.
Because of this, he wrote the book to try to get people to think about what could happen if governments kept becoming more controlling and totalitarian.
So, the book is kind of a warning about what might happen and a call for people to be careful so they can make sure they are not letting their governments move in this direction.
As other educators have commented, Orwell wrote 1984 as a warning against the dangers of totalitarian governments. With this in mind, it is also worth noting that 1984 highlights the impact of such political regimes on the development of language. Nowhere is this more apparent than in Newspeak, the official language of Oceania. While Newspeak is pure fiction, the idea that politicians use language to influence the public and to further their own agendas is not. In fact, this is an important and often-overlooked theme in 1984.
To put this into context, consider Syme's explanation of Newspeak in Part One, Chapter Five. In a conversation with Winston, Syme explains the purpose of Newspeak:
Don't you see that the whole aim of Newspeak is to narrow the range of thought? In the end, we shall make thoughtcrime literally impossible, because there will be no words in which to express it.
In other words, by taking direct control of the English language, the Party intends to manipulate the thoughts of the population. If they cannot express discontent, for example, then it follows that they cannot feel it. The purpose of this is to ensure that rebellion is kept to a minimum, thereby allowing the Party to flourish.
Orwell uses Newspeak to demonstrate the extremes of thought control but it has an important message for people living under all types of government: that language is instrumental in defining our liberty and freedom and we must never allow those in power to manipulate it.
As an extension of the points made above, we can look at the ways Orwell depicts life under a totalitarian regime. The de-humanization, constant intimidation and anti-intellectualism that besets every person living in Oceania stands as an inevitable set of symptoms of fascism/totalitarianism in government.
"Anyone who thinks subversive thoughts can be turned in by spies or by Big Brother, who monitors them through highly sensitive telescreens. If someone does not have the proper facial expression, they are considered guilty of Facecrime, so all emotions must be extremely carefully guarded" (eNotes).
Thus the message of the book is not limited to the notion that totalitarianism in government is bad. The message extends to the ways in which formal government attitudes can, and do, penetrate into the lives of those living in a society.
When leadership takes on such an extreme political philosophy (one that cannot allow compromise or question, e.g. "totalitarianism"), practical measures must be taken by the government to quell any and all resistance. The paranoia of such a system becomes, in itself, part of the system. People cannot be allowed to think freely, to act freely or to question the policies of the government - even privately.
"Such a pessimistic vision of the future serves a purpose, as Orwell knew. He wrote 1984 as a warning in order to make people aware that this type of society could exist if trends such as jingoism, oppression of the working class, and the erosion of language that expresses the vastness of human experience continued" (eNotes).
Orwell's message then is one of alarm. His message is that specific politics lead to specific social practices, which are unavoidable, inevitable and symptomatic of the nature of those politics.
Be wary of trading your liberty for your safety, we might say, in echo of Benjamin Franklin.
The message in the book 1984 was that the type of government portrayed will never be a benefit to society.
It takes away all individuality both in people's thinking and actions. It requires all people to be exactly alike so as to please the government and to stay within the prescribed constucts. It, also, can creates divisions between people with a sense of insecurity and suspicion.