When a creature misses you with a melee attack, you can use your reaction and expend one superiority die to make a melee weapon attack against the creature. If you hit, you add the superiority die to the attack's damage roll.
Immediately after you hit a creature with a melee attack on your turn, you can expend one superiority die and then try to grapple the target as a bonus action (see the Player's Handbook for rules on grappling). Add the superiority die to your Strength (Athletics) check.
When you hit with a weapon attack, you can expend one superiority die to add the total to the damage roll and the target must make a WIS saving throw (DC 8 + proficiency + strength/dex mod). On failure, it is frightened of you until the end of your next turn.