Wat is meant 由 roleplay?

你 make a character, 或者 choose one (like Harry Potter 或者 Voldemort) and act like them.
Say what they would say, and all that stuff.

It's where 你 take the "role" as the characters from Harry Potter, for example, and write them out with others in various situations. 或者 你 could use characters 你 made up yourself too.