From Bulbapedia, the community-driven Pokémon encyclopedia.
Dynamax (Japanese: ダイマックス Daimax) is a temporary transformation affecting Pokémon that was introduced in Generation VIII. Trainers who obtain a Dynamax Band are able to activate Dynamax for their Pokémon. Dynamax increases a Pokémon's size drastically, as well as changing the moves of the Pokémon and increasing their stats in battle, but can only be used once during a battle, and only for three turns. In addition, it can only be preformed in certain areas, such as special arenas in Pokémon Gyms.
All moves that a Pokémon knows will turn into Max Moves, which are more powerful than regular moves. The Max Moves that are learned are determined by the moves that the Pokémon knows.
Wild Pokémon in a Max Raid Battle start out in Dynamax form, and do not return to their original state during the battle. However, Trainer Pokémon do not start out in Dynamax form, and only one trainer Pokémon can turn into Dynamax form during the battle, and only for three turns.