A chain stop is a metal guide though which the chain passes on its way from the windlass to the bow roller or hawsepipe. It has a hinged finger which is manually lifted clear or removed entirely when the chain is to be deployed, but drops down into place and prevents the chain being let out when installed. The chain may still be retrieved (pulled), as the chain links then merely push the finger out of the way.
Chain stops are an important and inexpensive addition to a vessel's foredeck, but unfortunately are found missing on many – particularly production boats. When an anchor is set and the rode is 100% chain, even with the use of a snubber, the chain should be placed in the chain stop in order that the windlass is not expected to endure the force from the rode. If the anchor is a little stuck and is being 'powered' out using the boat, the chain should be held by the stop, not the windlass. Shock loads can bend windlass spindles, and of course a failed clutch would not hold the chain at all.
For similar reasons, a chain stop or other solution should be in place to guarantee the anchor is retained home once it is retrieved – tension from the windlass should not be relied upon for this.
A devil's claw is an alternative to a chain stop and emulates most of its functions, but it is not 'automatic' in the way that a chain stop is when the finger is present, by allowing chain to be retrieved but not let out (a non-return mechanism).