From Bulbapedia, the community-driven Pokémon encyclopedia.
Battle CDs are items found in Pokémon XD: Gale of Darkness. They are used to initiate Battle Sims at Realgam Tower. They allow Trainers to access simulated Pokémon battles that test their knowledge of advanced battle strategy. The player is given certain Pokémon to use and must defeat the opposing team in a set amount of turns.
The first time a player completes a Battle CD, he or she receives an item as a prize. Whenever a player completes all 50 Battle CDs, that player will be rewarded with 15,000 Poké Coupon points.