Multiple exposure is when you take two or more pictures and put them together into one picture! It's like taking a picture of a tree and then taking another picture of a bird and putting them both onto the same piece of paper.
You can do this with a special camera that lets you take more than one picture without having to wind the film. Or you can do it on your phone or computer by taking two or more pictures and then using a special program to put them together.
When you put the pictures together, you can make them blend together so they look like one really cool picture. Sometimes you can't even tell that there are two or more pictures in the same picture!
People like to do multiple exposure because it lets them be creative and make really interesting art. Sometimes it looks like a painting or a dream!