v: to pass from one point throughout a system and back to the starting point. For example, drilling fluid is circulated out of the suction pit, down the drill pipe and drill collars, out the bit, up the annulus, and back to the pits while drilling proceeds.
To permit the drilling fluid to go down through the drill string and well bore while drilling operations are temporarily suspended. This is done to condition the hole and the well bore before drilling continues, in order to prevent friction between the metal walls of the hole and the casing (that is, prevent metal rubbing against metal.)