You have mastered the art of exploiting openings in an enemy's defense.
• You get +2 to all attack rolls that you make outside of your turn (e.g. opportunity attacks). This includes things like grapple or shove checks, since grapples and shoves are also considered attacks.
• When an enemy within your reach misses an attack against you, you may use your reaction to make a single melee attack against the creature. The attack must be with a weapon (or fist, in case of an unarmed attack, a grapple, a shove or something similiar) that you did not attack with on your last turn. If this attack hits it disables the creature's ability to attack for the rest of its turn (interrupts multiattacks, further actions on the same turn cannot be used to take the Attack action e.t.c.).
It is worth noting that if you are wielding two weapons and only attack with one of them on your turn, the other one is still elegible for use with this feat. It must, however, be a melee attack, so if you are using this feat with a Hand Crossbow for example, it counts as an improvised weapon (1d4 dmg) as per the rules of the Player's Handbook. If you are wielding two weapons and neither was used to attack on your turn, you choose which weapon to attack with using this feat. If you are making the attack with a free hand, the attack can also be a grapple in addition to a regular unarmed attack, since both count as attacks and are normally attempted with the Attack action. The same goes for the shove action, and it can always be taken regardless (whether you have a free hand or not). Other actions might be passable if they could similarily be passed as an attack, DM's discretion.