back to the abs lights on my Peugeot 206cc

last week i posted that my abs light kept coming on .i was informed it might be something on the sensor ,so i had a look ,lo and behold my cv boot cover was loose and the brakes were covered in grease my question is i have put the cv cover back secured with a circlip.[ i think there called ? ] any how just behind the cv cover next to the brake disc there is a metal round thing with teeth on it and its snapped what is it and what is if for thanks