Source: Player's Handbook

You are an able physician, allowing you to mend wounds quickly and get your allies back in the fight. You gain the following benefits:

• When you use a healer's kit to stabilize a dying creature, that creature also regains 1 hit point.

• As an action. you can spend one use of a healer's kit to tend to a creature and restore 1d6 + 4 hit points to it, plus additional hit points equal to the creature's maximum number of Hit Dice. The creature can't regain hit points from this feat again until it finishes a short or long rest.