The EXP Charm is a new item that was introduced in Pokémon Sword and Shield in the Isle of Armor DLC. To get it you will have to buy the Season Pass and then go to Wedgehurst Station. Speak to the conductor to visit the Isle of Armor, where you will be treated to a new member of a Dojo located on the island.
She'll ask you to fight her, but after you defeat her, she'll try to convince you not to visit the Dojo where she trains, as she's afraid you'll take her attention away. You'll be able to see the Dojo on a hill in the background, so head towards it and you'll meet a new character called Honey outside of the Master Dojo.
Honey will ask you to come home, where you'll eventually meet her son Hyde. Hyde will give you the EXP Charm as a gift, which will then be placed in your bag. It's a good idea to give the EXP Charm to any Pokemon you have that's behind on your team, but you might want to save it for a specific Pokemon. If you don't want any spoilers, you should stop reading at this point and go visit Hyde to get your EXP Charm.
If you're still reading, you'll end up getting a special Pokemon by playing the DLC, the Legendary Kubfu. You can give him the EXP charm to increase the experience he gets from battles and quickly level him up to match the rest of your team.
