Zacchaeus was the chief tax collector and, like a lot of tax collectors of the day, he told people that the taxes were higher than what they were and took the rest for himself, which made Zacchaeus a wealthy man.
One day Jesus passed through Jericho; Zacchaeus wanted to see him but, like almost always, there was a HUGE crowd following him, and Zacchaeus was a short man. (It is estimated there was once a crowd of 15,000) So Zacchaeus climbed a tree so he could see Jesus. But Jesus told him to come down from the tree because he wanted to stay at his house.
Then Zacchaeus said he would give half of his possessions to the poor and if he had cheated anyone he would pay back four times the amount. I am not quite sure how he was going to do that, but it seems he did!