With every new Pokemon game, there is usually a new mechanic and for Pokemon Sword & Shield, it will be the Gigantamax.
Following up on what Pokemon Go has, the new Pokemon Sword and Shield games will also have Raid Battles and in these battles, you can actually take on some supersized Pokemon.
In Max Raids, the Pokemon that will be battling can Gigantamax and you will be able to capture them after. There will only be a few options in the beginning. From the 15th of November until January, you can encounter Gigantamax Butterfree in both versions. Those with the Sword version will also get to encounter Gigantamax Grednaw while those playing Shield will get to encounter Gigantamax Corviknight.
These Max Raid Battles will happen in dens in Wild Area. You can choose to take it on alone or join up with three other players. Depending on what game you have, you might also start off with a few free Gigantamax Pokemon.