A BioWheel is a kind of filter. It rotates by water flowing under it. They're pleated like the filters in a bagless vacuum cleaner.
I get the impression they're filled with the good kind of bacteria when you get them. As water flows through them, the good bacteria grows. Since part of the wheel is above water, it picks up a lot of oxygen in the air. Oxygen is like fuel for the good bacteria, so the wheel breaks down ammonia fast.
Over time BioWheel filters look disgusting, but that's a good thing. They say never replace it or you might have to recycle your tank.