Many times, for convenience, tennis pros will constantly say, "start low and finish high" when working on groundstrokes. The purpose of this is for the student to hit topspin (or "up" on the ball). Ideally, if a player could hit every groundstroke at or below their waist, then you would adhere to the statement "Start low, finish high." However, in the real world of tennis, the height that you make contact with the ball is always variable.
In fact, many balls bounce up very high and many players, especially juniors, are short. As a result, the high balls become nightmares. So, when receiving the groundstrokes near your waist, "start low and follow through high." When receiving at or over your shoulders, "start high and finish low." High follow through means that your racket hand finishes higher than your shoulders. Low follow through means finishing under your shoulders (or arm finishing touching your torso). This is how the pros play. This tip will really improve your power, control and consistency on the "high contacted" balls.
The problem for all players is, what to do with the slow, high bouncing groundstrokes. This tip will move you to a higher level of competency. Remember, on groundstrokes, you do not always finish high.
Good luck on the courts!