Zacchaeus was a tax collector, and he was very rich. He wondered what Jesus looked like, but he could not get near Jesus because of the crowd of people around Jesus. He couldn't look over their heads because he was a very little man. So Zacchaeus ran ahead of Jesus and climbed up into a sycamore tree so that he could see Jesus when we went along the road underneath the tree.
When Jesus got to the tree, he looked up and saw Zacchaeus looking down at him. Jesus called to Zacchaeus to come down out of the tree because Jesus wanted to stay at Zacchaeus' house that night. So Zacchaeus came down quickly and was very happy to have Jesus stay at his house.
When the people saw this they began to talk among themselves. They said that Jesus had gone to stay in the house of a sinner. Jesus said to them that he had come to save those who were sinners (Luke 19:9-10).