The new Pokemon Sword & Shield arrived with a few new mechanics and features including the Wild Area which is an open-world place where you can actually see wild Pokemons roaming free. Depending on the weather, you will see different Pokemons roaming the area some so powerful that you might want to avoid fighting with them first.
This is also where the Max Raids will take place. Max Raids are 4v1 battles where you and three other players or NPC battle one Dynamaxed Pokemon which is not only large but also powerful.
These raids can be done on your own with the three other trainers being NPC but it will be easy if you actually have one real player among the group. To start the raid, you will need to enter the raid den, if there is a red or purple beam of light shooting out, there is a Pokemon inside. You will also see a silhouette if the Pokemon so you can prepare your lineup.
To trigger a raid encounter in an inactive den, you can use the Wishing Piece to spawn a raid Pokemon. At five-star tier, you will also encounter Gigantamax Pokemon.