A deflection in basketball is when a defender hits a piece of the basketball either on purpose or inadvertently. In this situation, they do not get immediate possession, but they do create a loss ball off the deflection.
Many coaches will tell you to play active defense, or hands up defense, this just means you should keep your arms waving in the air in front of the person with the ball. Not only will this make passing or shooting much more difficult, you can also cause a deflection. This means that the offense tried to make a pass and you tipped it with your arm or hand, creating a loose ball. This can lead to a steal or a turnover, which can help your team score points and win the game.
Playing hard on the ball defense can cause frustration for the opposition, because for every pass, shot, and dribble, they have to work a little harder. By constantly harassing and interfering with the ball carrier, the offense can become more tired and turn the ball over more throughout the game.
Many teams have defensive strategies surrounding deflections and hard on the ball defense. Some college teams even run a full court press defense, meaning they pressure the ball all the way on the other side of the court. The point of this is to try and cause turnovers by the opposition through deflections or other means. Although deflections are not a standardized statistic, many coaches including the former coach of the Indiana Hoosiers, Tom Crean, look at the statistic and use it to mold their defense.