A watermark is a notation on your photo that claims ownership (not copyright). Many photographers will add a watermark (name or logo) on any image they post online so that when it is shared, credit is still given. In this example, the watermark is usually small and off to the corner of the image.
A watermark is also used on proof images that a photographer gives to a client so that the client will not use these smaller images. In this example, the watermark is usually larger and in the middle.