While you can eat food to refill your health in Kingdom Come Deliverance, there is a much easier and fast option:

Simply sleep in a bed and it will give you 10 health per hour of sleep!

During some main quests you can't sleep. However, as you progress the game this option will become more readily available. You can also sleep in other people's beds, just don't let them catch you.

If you are bleeding you must use a bandage to stop the bleeding. However, sleeping also cures most of these negative status effects (such as food poisoning). Alternatively, you can go to a bathhouse to refill your health but this costs 30 coins. There are no medkits or anything so you either have to sleep or use potions or food or go to a bathhouse, those are the only options for refilling health.