The clutch release bearing is responsible for connecting the clutch fork to the clutch cover and disengaging the clutch.
This item should be replaced during clutch replacement. Additionally, many aftermarket clutches do not come with a release bearing and thus it must be purchased separately.
Signs of release bearing failure include whirring and grinding noises which often vary with the position of the clutch pedal. WTFauto recommends Toyota genuine bearings for their excellent quality and lifespan.