A round character is a major character in a work of fiction who encounters conflict and is changed by it. Round characters tend to be more fully developed and described than flat, or static, characters. If you think of the characters you most love in fiction, they probably seem as real to you as people you know in real life. This is a good sign that they are round characters.
A writer employs a number of tools or elements to develop a character, making him or her round, including description and dialogue. A character's responses to conflict and his or her internal dialogue are also revelatory.
How do you go about creating a round character rather than a flat one? Creating truly believable characters takes time and thought, of course; you can start by answering these questions about your main character.