The Ghost of Christmas Past was the first of the three spirits to visit Ebenezer Scrooge after he was visited by Jacob Marley.
This ghost had a few different purposes in the story. One of those purposes was to show Ebenezer how necessary it was that he change his ways. Another purpose of this ghost was to show the reader how Ebenezer came to be the way he is presently. The idea here is that most of the things that happened to Ebenezer that made him bitter happened during the holiday season.
Some of the things the ghost showed him were his old boarding school. This reflects a lot of negativity because his father abandoned him. In addition, he was shown his fiancee Belle, and how he didn't protest when the engagement was broken off due to his obsession with money. He was also shown how she ended up finding happiness with another man.
In this "Stave," the ghost takes Scrooge back to look at his past. It is showing him how his greed has made him miserable.
It shows him how he happy he was when he was allowed to come home from school for Christmas. It shows him how happy his old boss made people by giving a dance at Christmas. And it shows him how happy his finacee was after she dumped him and married someone else -- after telling him that his obsession with money was why she was dumping him.
These things help to show Scrooge that his life could be much better if he were less greedy.