A grave is the place where a body is buried. Usually, a grave is dug in the ground, and a stone with information about the deceased person marks its location.
Grave is used as an adjective, too. It can describe something serious, or of great importance. If a situation is grave, it is serious and sad, like when a loved one is very sick. Grave can also describe causing fear or anxiety. If you are in grave danger, your personal safety is threatened. If you have a grave personality, you are solemn and dignified and don't joke around very much.