Max Raid Battles are a multiplayer feature that utilises the Dynamax Feature. In the Wild Area, you'll often find areas where a Dynamaxed Pokémon can be found. This Pokémon, while Dynamaxed, will not revert back after 3 turns.
To defeat it, you have to play with up to 4 other players to battle it, with one player being able to Dynamax their own Pokémon. This can be done locally or online, but if three more players can't be found, Support Trainers will be added, controlled by NPCs.
The Pokémon you'll find in Max Raid Battles change every day and based on the weather, for example Steenee appears in Sunshine and Lanturn in Rain. Some Pokémon are also exclusive to Max Raid Battles. Max Raid Battles also often have limits such as the battle ending if 10 turns have passed