Imagine you are drawing a picture and you want to make sure nobody else takes credit for it. So you put your name on it somewhere, like in the corner or the middle. That's a watermark!
A watermark on a digital file is kind of like that. It's a little mark or design that's put on the file to show who made it or that it belongs to someone. It's usually harder to see than your name on a drawing, so it doesn't get in the way of people looking at or using the file.
There are different ways to make a watermark, but one common way is to put a faint image or text over the top of the original file. This makes it clear that it's a special version of the file with a watermark on it.
People use watermarks on all kinds of files, like photos or documents, to show that they are the owners or creators. Sometimes, watermarks are used to make it harder for other people to steal or misuse the files.