A Constant-Velocity (CV) joint transmits engine torque to the wheels smoothly. Standard universal joints speed up and slow down twice per rotation when bent at an angle, which causes vibrations in steering shafts. CV joints solve this by using steel balls sitting in curved grooves inside an outer housing. This allows the axle to pivot up to 45 degrees while spinning at a perfectly constant speed, which is necessary for front-wheel-drive wheels that must steer and travel over bumps.
A rubber boot filled with specialized grease seals the joint. If this boot splits from age, the grease flings out and road grit enters. The grit acts as sandpaper, grinding down the steel balls and grooves. You will hear a clicking noise when turning, indicating the joint is worn out.