The EXP Charm is a new item that was introduced to Pokémon Sword and Shield in the Isle of Armor DLC. To get it, you will need to purchase the Season Pass, then make your way to Wedgehurst Station. Talk to the conductor to visit the Isle of Armor, where you will be treated by a new member of a Dojo located on the island.
She will ask you to battle her, but after you defeat her she will try to convince you not to visit the Dojo she trains at, as she is afraid you will take attention away from her. You will be able to see the Dojo on a hill in the background, so just walk towards it and you will meet a new character called Honey outside the Master Dojo.
Honey will ask you to come inside, where you will end up meeting her son Hyde. Hyde will give you the EXP Charm as a gift, which will then be placed in your bag. It is a good idea to give the EXP Charm to any Pokémon you have that is lagging behind your team, but you might want to hold on to it for a specific Pokémon. If you don’t want any spoilers, you should stop reading at this point, and go and visit Hyde to get your EXP Charm.
If you are still reading, you will end up getting a special Pokémon as you play through the DLC, the Legendary Kubfu. You can give him the EXP Charm to increase how much experience he gets from battles, and rapidly level him up to match the rest of your team.