To sum something up means to summarize it or share only the very important details. For instance, if you borrow your friend's car and then wreck it in an accident, you don't go into all of the details of what happened that led up to the wreck. Instead, you start by simply stating that the car was wrecked in an accident. Your friend will ask questions to get the additional details that they believe are necessary. Communicating this way tends to be more effective.

to say a main idea

If you were talking to someone and she said she is busy, sum it up. She was telling you to hurry and say what you have to say so she can get back to what she was doing.