A chain stopper reduces the tension caused by the pull of the chain on your windlass/anchor winch.
Normally when you are anchored without using a stopper, the chain from your rode pulls almost completely at your windlass. During rough conditions, this strain can be enormous and take a toll on the windlass' gear train and breaking mechanism. When you install a chain stopper in between your bow roller and your windlass, the chain stopper bites down on the chain when it tries to pull, moving the strain of the chain from your windlass to the chain stopper.
These stoppers are fairly cheap (around $40-50) and should definitely be considered if you're using a windlass.
