RollQuest summary of the SRD 5.1 rules, not a quotation.
Pick one: Evasion: when you make a Dexterity saving throw to take only half damage, you instead take no damage on a success and only half on a failure; Stand Against the Tide: when a hostile creature misses you with a melee attack, you can use your reaction to force that creature to repeat the same attack against another creature (other than itself) of your choice; or Uncanny Dodge: when an attacker you can see hits you with an attack, you can use your reaction to halve the attack’s damage against you.
RollQuest ships the D&D 2014 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.