Challenging is when you receive an easy ball in the point and you should be able to hit a clean winner, but you do not. Instead, you hit it near your opponent and give them one more chance to continue the point or end it. Many times, this restarting of the point, allows your opponent to get back in the point or they may be able to end the point.
A good way to stop this challenging is to imagine your opponent is sitting in a chair. If you hit the ball close enough to allow your opponent to hit it without leaving the chair, you are challenging. If you hit the ball, forcing your opponent to leave the chair to touch it, this is called hitting to the "opening" -- not challenging.
Unfortunately, some very exciting points are a result of players mindlessly challenging by hitting the ball too hard. You don't go to the net to rally, you go to the net to stop the point. At higher skill levels, players are not afraid of power. They can actually use it against you. Placement, finding openings, is more important than power.
Remember, challenging is what you do when you receive a difficult ball and you can only continue the point. If possible, quit challenging when you receive the easy balls. Find the opening in the court, hit to the opening and quit challenging. Get your opponent out of their chair. The best time to challenge is in the warm-up or in the practice session. Once you start keeping score, quit challenging.
Good luck on the courts!