Whenever a creature attacks you in melee, and you have a single melee weapon equipped, you may use your reaction to attempt to strike back at the attacker. Make an attack roll, if your roll is higher than the attacker, you parry their blow and strike with your roll. The attacker then has the option to repeat this process. If they miss, your attack hits. If they succeed, their attack hits. If you have two melee weapons equipped, you can either attempt to parry, and attack twice, or you can choose to block the attack with both weapons. Make an attack roll, if you succeed, you can cause their attack to not land and be blocked by your own. The same is with a shield. If you have a shield equipped, you can stop their attack if you get a higher roll. If you have the shield master feat, you can make an attack with your shield similar to that of a weapon.