Does what is says on the bottle! It is used instead of a wash as it helps to prolong the life of the fix and stops the developing process straight away. You must be careful not to contaminate the developer with stop, it will destroy it.
Stop can be obtained in two types of concentrated solution. One is odorless made from citric acid and the other acetic acid with a vinegar smell which can be quite pungent if used for prolonged periods in the darkroom, when processing prints in trays. Both have a colour change indicator added to the solutions so you can tell when it is exhausted.
Dilute the concentrated Stop one part solution to nineteen parts water (or as instructed on the bottle.) say 50 mls stop to a litre of water with a process time of one to two minutes, you will not gain anything by extending the stop time it is most active in the first few seconds. Agitate for the first thirty seconds, this ensures the developer has been completely deactivated, then ten seconds in every minute if you stop for two minutes.