Apply foods to eat them immediately, restoring your hunger stat. Eating food when low on hunger also proves a very minor healing effect as a bonus. Food items can be examined to see how many nutrition points they restore. Waybreads tend to be one of the best items out there in terms of nutrition (+500 food) compared to its weight.
If your hunger his 0 out of 1000, you will first attempt to automatically eat food a random food item you are currently carrying. If you're not carrying any food, you will begin starving, taking damage to your health rapidly. Note that if your character is sitting in game and you're away from the keyboard, make sure you're actually carrying food and not just standing on a pile of food, as the game will only attempt to eat food you're physically holding.
Items with +Sustenance such as an Amulet of Sustenance will significantly slow down your metabolism and thus the rate at which food is consumed. Items with +Regeneration or +Magic that increase how quickly you recover health and mana tend to also raise your metabolism, causing you to go through food quicker.
Eating enemy corpses is also an option to regain nutrition points, but note that some acidic corpses such as the ones left by Gaeolotrolls are unsafe to eat and deal damage when eaten. Dragon players that eat corpses can gain permanent resistance bonuses depending on what elements the enemy resisted and how powerful the enemies are relative to the Dragon.
Undead characters, such as characters that started as the wraith race or ones that started as a different race and then chose to worship The Devourers, do not require nutrition and cannot starve to death (though being undead has drawbacks such as being vulnerable to Banishment spells).
The Create Food spell that Summoning has access to can allow you to create emergency rations if you're in the wild and didn't bring sufficient food with you.