I’ve heard editors say how important it is for a character to be well rounded. What makes a character well rounded?

from The Beginning Writer's Answer Book, edited by Jane Friedman

Well-rounded characters are distinctly individual; you come to understand their motivations, flaws, emotional traits, and other distinctive qualities. These can be related through a character’s actions, his reactions to situations and other characters, dialogue, and also through narrative. A flat character, in contrast, usually carries only one distinguishing trait or might act predictably or according to stereotype.