Throw out bearing: When you step on the clutch pedal you're pushing the friction plate away from the clutch disc and flywheel to disengage the clutch.
Of course this is a rotating component. The component that pushes on this when you step on the clutch pedal is stationary. There has to be some way of pushing a stationary component against a rotating component. The throw out bearing is what is between the stationary component and the rotatiing component. This bearing can wear out. When it does it can make noise.