Okay, so a long long time ago there was a man named Matthew. He wasn't a very well-liked man because he collected taxes from people, which meant that he took money from them. Imagine if someone came to your house every month and took some of your pocket money, you wouldn't like them very much either.
One day, Jesus, who was a very famous and kind man, came to Matthew and said "Follow me". At first, Matthew was surprised because he didn't think Jesus would want to be friends with someone like him. But Jesus didn't care that Matthew collected taxes, he saw that he was a good person deep down inside.
So, Matthew decided to follow Jesus because he knew that he was someone special and different from anyone else he had ever met. He wanted to learn from Jesus and become a better person.
That's the story of the calling of Matthew. It shows us that no matter what we've done in the past, we can always change and become better if we choose to follow Jesus.