A legion is a horde or a large number of people or things. It was originally a term for a military unit.
In Roman times, a legion was a large unit of men in the army, and the word is still used in many militaries. However, it also means a whole bunch of people doing anything. If someone has many fans, you can say legion of fans or legions of fans, but either way, that's a lot of fans. There are legions of stars in the sky. When you see legion, you can be sure there's an awful lot of something.