A composite portrait is like when you take two or more pictures and combine them into one big picture! It's sort of like creating a Frankenstein monster picture, but way less scary.
Let's say you want to create a picture of a superhero. You could take a picture of someone with a really cool mask, and then take a picture of someone with a strong body, and then take a picture of someone with really long arms. Then, you would put all of these pictures together, so that the mask person's head is on the strong body person's body, and the long arms person's arms are on the body too!
That way, you have a new picture that looks like a hero who has a great mask, strong body, and long arms. You can use this technique to create all sorts of fun pictures, like a picture of a celebrity with a body made out of food, or a picture of an animal with a person's face. The possibilities are endless!