A flat character is one-sided. The reader doesn't know much about them, and they are generally there to move the story forward. An example of this in your life might be your bus driver. You know what he/she looks like and how he/she may have reacted to a given situation on the bus, but you really don't know them. You don't know what they think, what their background is, what motivates them.
A round character is one that is fully developed. The reader knows what the character thinks and feels. He knows his background and can predict what the character will do in a given situation. An example of this in your life might be your best friend.
Mayella is a flat character. We only meet her in Chapter 18, and then she is gone. She responds to the attorneys' questions during the trial. She is afraid of what Atticus will do to her on the stand, so she is reluctant to talk. We know nothing of her. We do not know what she thinks, we do not know how she is going to act, and actually, we only see her through Scout's eyes. We never get to meet Mayella on her own.