Spit don't rinse is the advice we give to all patients to adopt when brushing their teeth. Put simply, we encourage everyone to brush for 2 minutes with a fluoride toothpaste, and when finished to spit out, but not rinse or drink directly after brushing.
There was a time when the advice was to rinse with water or mouthwash after brushing, to flush out all the plaque, however, the most up to date evidence recommends not rinsing out, so that a film of toothpaste is left on the teeth after brushing. By not rinsing out, the teeth are exposed to the toothpaste, and most importantly, the flouride for longer.  This helps to strengthenand repair the teeth. 
This is especially important last thing at night, when no food or drink is taken, so allows the useful ingredients in toothpaste to keep working after brushing has finished.
Many patients ask about use of mouthwash after brushing, and where they fit in with the spit don't rinse message. Our advice would be to try to use a mouthwash at a different time from brushing.  For example use mouthwash when you get up, and brush your teeth before leaving the house in the morning.  Or to use mouthwash after your evening meal, and to brush with toothpaste last thing at night.  This has the advantage of increasing the frequency with which the teeth are exposed to fluoride, and will help to cut the risk of decay and cavities.  So remember, after brushing - Spit Don't Rinse!