While out adventuring, heroes can take Field Rests to regain HP.
Catch Breath. Requires at least 10 minutes to tend to your injuries. Expend any number of Hit Dice one at a time (roll them and add your STR to each), and regain that many HP.
Make Camp. If you rest for at least 8 hours with food and sleep, take the maximum value for each Hit Die you expend instead of rolling. Add your STR to each Hit Die as usual.