A groundstroke in tennis is a forehand or backhand shot that is executed after the ball bounces once on the court. It is usually hit from the back of the tennis court, around the baseline.
A tennis player whose strategy is to trade groundstrokes with the opponent is termed a baseliner, as opposed to volleyers who prefers to hit volleys near the net.
There are many factors that may define a good ground stroke. For example, one ground stroke may use topspin and another underspin. Both can be effective for different reasons having to do with depth, opponent's strength or weaknesses, etc. Some characteristics of ground strokes are: depth (how close the ball lands to the opponent's baseline), speed (how fast it travels in the air), pace (the ball's behavior after it bounces on the opponent's side), trajectory, and placement. If a "good ground stroke" is to be played, it would generally have a combination of the above characteristics to produce a shot that is difficult for the opponent to return. Generally, a ground stroke that lands deep and in the corner of the opponent's court will make it more difficult for the opponent to return the ball. However, this is somewhat arbitrary and depends on the opponent and stage of the point being played. For example, a short angled shot, a moon ball (very high trajectory), an off pace shot, etc., may prove effective against opponent A but not opponent B.