the defensive concept of guarding another player

Defending by staying close to an opponent.

Covering or guarding an opponent.

guarding an opponent. To "mark up," for example, is to guard opponents for throw-ins.

Guarding your opponent

when defenders stick closely to opposing players, to prevent them from gaining possession

Being within a stick's length of an opponent.

The position taken up by the defensive player which is designed to keep his/her offensive opponent from either receiving the ball or being an effective offensive threat.

To mark means to take defensive responsibility for an attacking player. If I am marking you, my first priority is this one player. This would be similar to guarding a player in basketball.

Guarding and defending a player to prevent him from advancing the ball towards the net, making a pass or getting the ball from a teammate.