Boars are hostile wildlife found in the Meadows biome in Valheim. They will squeal and charge at you if you get too close. These little chubbers drop tasty meat and leather scraps that you need for (what feels like) half the recipes in the game.
You will almost always find boars in groups of three, so if you only see two, the third one is coming to tusk you in the rear. There is also a boar runestone that will be surrounded by more than three boars.
You can also tame boars by feeding them vegetables.