Source: Player's Handbook

You have mastered techniques to take advantage of every drop in any enemy's guard, gaining the following benefits.

• When you hit a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn.

• Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach.

• When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn't have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.