When someone synopsises something, they write a short summary of it.
If someone synopsises something, they are usually giving a quick explanation of it.
Examples:
She synopsises the story for her friend.
Can you synopsise the movie for me?
He synopsises the book in just a few sentences.
I synopsise the article for my class.
The teacher synopsises the lesson for the students.
He synopsises the plot of the play in a few words.