When you are wielding a finesse weapon with which you are proficient and another creature hits you with a melee attack, you can use your reaction to add your proficiency bonus to your AC for that attack, potentially causing the attack to miss you.  If this causes the attack to miss, you may move 5 feet in any one direction without provoking an attack of opportunity.  If you are wielding two weapons then you may instead make a single attack with your offhand weapon.

Cannot be wearing medium armor, heavy armor or a shield.