Aside from winning and losing, the goal in tennis is to keep the ball in play longer than your opponent can. To do this you use strokes: forehands, backhands, serves, overheads, etc.
"Keep the ball in play" means it lands inside the playing court of your opponent. Hence, the playing court becomes your target. When your game progresses to where just hitting the ball over the net and into the court is no longer a challenge, you're ready for smaller targets.
When you choose a target (or series of targets if you REALLY want to be a good player) you are using strategy or "tactics". Tactics are the plan to be executed by your strokes. Said another way, your strokes are for hitting your targets. The better your strokes, the smaller the target you can hit.
For your strokes to improve, you need to choose targets that are challenging. Don't settle for merely getting the ball over the net. Your game is only as good as your ability to place the ball where you intend it to go.
While you may not have a down the line backhand for example, it won't develop until you choose that as a target and try to hit it. If you think "I'll choose that target when my strokes get better", that's like saying "We'll go to the moon when a space shuttle shows up!"
The goal is the catalyst for its attainment. Tactics precede stroke production - because you have to choose the target before you can hit it.
So, choose a target every time you hit the ball.
Sounds simple and it is.