We repeatedly are told we have to move to hit the ball - but it's just as important to move after you hit it.
Too many players hit an acceptable shot, then stand around admiring it and wind up losing the point. There is no time in an action game like tennis to applaud yourself and still get in position for the next shot.
And you always have to assume there will be a next shot.
If you are playing at the baseline, shift immediately back toward the center of the court after you make a shot. Take a balanced stance so you can move in either direction if the point continues.
At the net, quickly get your racket back up in front of you after you hit a volley. Then you'll be prepared for the ensuing volley.
Every shot in tennis has to be followed by some fast footwork if you want to play well. Hit and move, hit and move, hit and move.