In basketball, there is a specific stance that defensive players use to maximize their ability to guard the opposition. Maintaining a proper stance allows the defender to stay in front of the ball handler and contest every shot attempt, ideally forcing a miss.
By bending slightly at the knees and assuming a squat-like position, defenders are able to perform lateral (side-to-side) movements. This technique enables them to account for every direction that might occur on behalf of the ball handler, as defenders can quickly shuffle left/right to cut off offensive players that are attempting to dribble past them.
In order to be prepared to steal the ball or prevent the ball handler from having an open look at the basket, defenders need to maintain what is commonly known as 'active hands.' Keeping both hands outstretched to the side as opposed to hanging near the floor helps defenders deflect ill-timed passes and distract offensive players when they attempt to shoot the ball.
Defenders need to be ready to mimic the ball handler's every move, otherwise they risk being faked out and giving up an open shot. Placing a majority of their body weight on their toes as opposed to standing flat-footed prevents a defender from being a step too slow by allowing for heightened reaction time.
Developing a strong defensive stance and translating it to game action requires a great deal of practice. Carefully observing some of the NBA's best defenders (i.e. Danny Green, Patrick Beverley, Draymond Green) and making mental notes of their positioning as they are guarding an opposing player is a great place to start. Once you have a grip on what a proper defensive stance should look like, hit your basketball court to perfect your craft. Don't be afraid to challenge yourself and practice by guarding the best/quickest ball handler that you know, as this is the only way to become a truly great defender.