Gardening is the commentators' term for the action of a batsman engaged in tapping down spike marks or loose pieces of the surface of the pitch with the toe end of the bat.

Many batsmen will do this purely as an aid to concentration, even when there is nothing obviously amiss with the surface.
Batsmen are entitled to engage in a little innocent gardening, but they are not allowed to damage the pitch - beating it ferociously with the bat, for instance, is certainly not permitted.