When you are damaged, press and hold Triangle/Y to heal yourself. The effectiveness of your healing depends on the type of food you are using and the different skills you have learned.
To view your available food recipes and set your quick heal options, press D-Pad Left. Constantly gather meat and plants to keep the necessary ingredients on hand for food crafting.
Keep helping Sayla to unlock new recipes, which both heal you and add temporary boosts to your abilities. For a full list of food check out the Food Recipes chapter.