A hairline is the edge of a persons hair especially near the persons forehead.
When someone has a receding hairline it means that the person is becoming bald in the area where the persons hairline is.
Usually a receding hairline is hereditary and usually starts at the end of puberty.
It's usually normal for some people to have a receding hairline but other times it could just mean that you're on your way to becoming bald eventually if you don't use some hair regrowth products to help with the receding hairline.