The air stone fits in the base of the biOrb underneath the filter cartridge. It breaks up the flow of air into the aquarium, splitting it into a mass of fine bubble. It also reduces noise from the air pump. 

Over time the tiny pores in the air stone can become blocked by dirt and lime scale preventing air from getting through properly. Depending on your local water supply and the conditions in your biOrb an air stone can last anywhere from 3 months to 3 years. 

Changing the air stone from time to time is advisable.