Lol, do not worry, I have photoshoped a few pics and I am already pretending to be an expert Watermark is like a transparent mark (can be as a text, anything you want, starting from your name)that you are able to paste onto your photo to prove it is yours. When it is transparent, it enables you to put it anywhere in the photo, the more unusual place, the more difficult it is to get rid of it for the potential thieves.
Anyways, have a look online for a better definiton. I do not think I am making sense tnight