For a fanfic set in a high school, five times a student gets a normal grade on a test, but then one time they ace a really difficult exam that no one expected them to do well on. This could be an instance of a '5 times 1 time fanfic'. It shows a pattern of multiple regular occurrences followed by one standout event.
In a fanfic about a magical world, perhaps five times a young wizard uses a simple spell to solve small problems like lighting a candle or opening a locked door. And then one time, they use a very powerful and dangerous spell to save the whole wizarding school from a dark force. This kind of pattern could be what is meant by '5 times 1 time fanfic'.
Sure. Let's say it's a fanfic based on a popular TV show. Five times the main character has a normal conversation with their sidekick, and then one time they have a really deep, emotional heart - to - heart that changes their relationship. That could be an example of a '5 times 1 time fanfic'.