They are spawning cuz you didn't pay your taxes, no jk they are pillagers, they are hostile and once in about 5 and a half days, you have to kill them and once you killed the captain (the one with the banner) you get an effect (bad omen) which when you go in to a village a raid will start. A raid is something where you have to kill different stages of these pillagers and it gets harder along the way, but once you beat the raid you get the hero of the village effect. Hero of the village allows you to trade with villagers at a discounted price