I have read questions that allude to spending hit dice on healing in 5th edition, and seen twitch streams of games where this mechanic is employed, but have not been able to find it in the Dungeon Master's Guide or Player's Handbook. I am probably overlooking something obvious in the indices (I have looked, though :).
Where are the rules for spending hit dice on healing in 5th edition?
NB: neither the Player's Handbook nor the Dungeon Master's Guide indexes pages on hit points, or hit dice, and the Player's Handbook-indexed note on healing on page 197 mentions nothing about the hit dice mechanic.
Best Answer
Hit Dice acts as a way of recovering your lost health over a short rest. This is a self-heal that scales as your character levels up (PHB pg. 186, or here in the basic rules):
There are also a few variant healing rule options in the DMG that the DM can choose to use instead of the normal resting rules.
One is that you must first use a healer's kit to bandage up the wounds before you can spend any hit dice (DMG pg. 266):
The other optional rule affects long rests. Normally, you regain all your hitpoints after a long rest, as if by magic. A variant rule makes you spend your hit dice to heal up instead, making being fully healed more difficult (DMG pg. 267):