Summarising means to give a short version of something, like a story or a book.
Summarising is helpful for understanding things quickly. It's like telling the most important parts of something.
Examples:
I am summarising the story.
She is good at summarising books.
They are summarising the movie.
We are summarising the long article.
Summarising the whole book in just one page is hard.
He is summarising the big story for us.