So kiddo, a composite application is like a toy with many parts that work together to make it fun to play with.
In the computer world, it's like having a bunch of different software programs that work together to do one big task. For example, imagine you need to write a letter and send it to your friend. You could use a composite application that includes a writing program, an email program, and a printer program.
When you click the "send" button, all the parts work together to first save your letter, then attach it to an email, and finally send it off using your email program. You don't have to do all those things separately yourself, the composite application does it for you.
So just like a toy with many parts, a composite application is a bunch of programs combined together to do something big and cool.