Sequel is like a storybook that comes after another storybook you've read. So, imagine you read a book about a little pig who built a house of straw, and then you read another book about the same little pig who built a house of sticks. That second book is the sequel because it continues the story of the first book.
In the same way, if you watch a movie and then a new movie comes out later that tells more of the story, that new movie is the sequel. It's like a continuation of the first story.
Sometimes, a sequel can have the same characters or setting as the first story, but other times it might have new characters or take place in a completely different place. But the important thing is that it builds off of the original story and makes it even more exciting!