Jesus started by telling the story to men called tax collectors and Pharisees. These were men who thought they were more important than everyone else and they weren't very nice. They were whispering about Jesus and how he shouldn't eat and talk with sinners.
It's funny they would say that because we all are sinners, we all do things we shouldn't. The tax collectors and Pharisees thought that they were better than everyone else. In fact, they didn't think they had to repent or say sorry to God for their mistakes.
Have you ever felt really bad for something you've done? Maybe you've hit your brother or sister and afterwards you wished you hadn't. When you really feel sorry for something and tell that person you are sorry, that is what we do when we repent.
Just like with our family we need to feel bad and say sorry to God when we've done something wrong. The Pharisees and tax collectors didn't think that they had to do this because they were so important.
Of course Jesus knew what they were thinking, so he told this parable:
Suppose one of you has a hundred sheep and loses one of them. Does he not leave the ninety-nine sheep and go find the one sheep until he finds it? And when he finds it, he is so excited he carries the sheep all the way home. Then he phones all his friends and neighbors and has a party to celebrate the sheep that was found.
Then Jesus told them what the parable meant: Just like there was a big party over the one lost sheep, there will be a big party in heaven over just one sinner who repents than over ninety-nine people who think they are perfect and don't need to repent.
Just imagine, when you say you're sorry to God, all of heaven has a big party just for what you've done! God is much more excited when we say we're sorry than when we act like nothing has happened and don't think we need to repent at all.