A bloodstain is a mark left by blood on something — a shirt, a sheet, a floor, a wall. It can be a tiny spot or a bigger patch. People usually mean the mark after the blood has sat there, often after it has dried; they do not usually use this word for blood that is still wet and running. You hear it in ordinary talk when someone has cut themselves, and a lot in police and crime stories. It is a plain, serious word — not slang, and not fancy.