XL candies in Pokémon Go are a special type of candy used to power up Pokémon beyond the previous level cap of 40, allowing them to reach up to level 50. You can start receiving and using XL candies only once you reach Trainer Level 31. Key points about XL candies in Pokémon Go:

• They allow leveling up Pokémon from level 40 to level 50.
• XL candies are specific to each Pokémon species (e.g., Magikarp XL Candy can only be used on Magikarp).
• XL Rare Candy can be used on any Pokémon above level 40 and converts into that Pokémon's specific XL candy upon use.
• It takes 296 XL candies, along with Stardust and regular candy, to power up a Pokémon from level 40 to 50.
• XL candies can be obtained from catching Pokémon (especially evolved, Legendary, or Mythical ones), trading, hatching eggs, walking with a Buddy Pokémon, or converting regular candy (100 regular candy = 1 XL candy).
• XL candies are rarer and harder to get than regular candies.
• Using XL candies boosts the CP and stats of Pokémon beyond the previous level cap.

In summary, the main purpose of XL candies is to enable trainers to power up their Pokémon beyond level 40, increasing their strength and combat potential significantly.