Dragonborn heritage: Constitution and Defense (5e Feat)
From D&D Wiki
Dragonborn Heritage: Constitution
Prerequisites: You must be a Dragonborn
As a Dragonborn, your heritage usually has it's root in magic, but it also has it's root in high constitution and a naturally high defense in this case, you have tapped into one part of that more physical heritage.
Tapping into the power that lies waiting inside you, you gain +1 to constitution, a maximum of 20 and as a bonus action you may tap into a small pool of temporary hit points relative to half your level and you may use this once per battle. Using it more than 5 times a day however will push your body further than it is meant to be pushed and can give you a point of exhaustion.
Alternatively, you can tap into the hidden defensive ability you have. Your scales harden and your skin becomes rough, no longer needing armor your AC, when not wearing armor of any kind, becomes 10+your constitution modifier+your strength modifier. As a bonus action, you can give up the charge of your breath weapon and wreathe yourself in your element, giving any enemy disadvantage on their attacks against you until your next turn. This ability can be used as a substitute for your breath weapon, and rules for that still apply as normal.