When someone summarises something, they give a short version of it.
If someone summarises, it is usually helpful because it makes things easier to understand.
Examples:
She summarises the story for her friend.
Can you summarise the movie for me?
He summarises the book in just a few sentences.
The teacher summarises the lesson at the end of class.
The summary of the story summarises everything in just a few words.
The news anchor summarises the main events of the day.