Dynamax Crystals are items given to players during certain events or through pre-ordering Pokémon Sword and Shield.
These crystals allow players to summon specific Pokémon as Max Raid Battles within the Wild Area. Some Dynamax Crystals include the infamous and weirdly-named ☆And428 and ☆And15 Crystals that summon Jangmo-o and Larvitar.
Once you have the crystals in your possession, you need to head to the Wild Area to use them.
Find any empty Den, the red glowing flower-like objects scattered around the area that usually house Raids, and load up your Item Menu. Choose the crystal you want to use and it’ll activate the Den, bringing with it a new Raid for you to take part in.
After this, it’s a simple case of battling the Raid and taking down the Pokémon until it’s ready to be caught. You’ll only get one shot at adding the Pokémon to your team, so be sure to make it count.