A mournful person is someone who is full of sorrow, like a little girl who has just lost her new puppy.
Mournful is a word that is often associated with someone who has experienced the death of someone close to them. A person would feel mournful if their loved one had just died. Mournful can also describe the expression of sad feelings, like a person dressed in black and sobbing into a handkerchief. You would not be mournful if you had just failed a quiz, but you would be mournful if your twenty-year marriage had just failed.