What is an alternator belt tensioner and does it lock up?

To the best of my knowledge an alternator belt tensioner is just a fancy way to refer to the pulley that the alternator belt goes around yes they do lock up if your having that problem you've probably already shredded your alternator belt and youll need to purchase a new belt and a new pulley now depending on what kind of vehicle you have you may have to rent a special tool from the auto parts store to remove and or install your pulley which if im not mistaken you will want to ask for a pulley tensioner remover i hope i helped some!