It is called a fill as you are “filling in” the basic rhythm of the music with an alternative sounding pattern using different parts of the kit such as tom toms and cymbals.
Generally speaking you should be aiming to come out of the fill at the same speed as you went in so you should be counting time for the duration of the fill. Play each stroke of the fill cleanly in the centre of the drums and at a consistent volume. Avoid rim and stick clicks and the tendency to” trail off “in volume and execution through the fill as you concentrate on getting back into the groove. Play it like you mean it or don’t play it at all