From Halopedia, the Halo wiki
The credit, abbreviated as "cR," is the standard currency of the Unified Earth Government. As of 2558, the Credit is still in use. Credits can be spent in the form of "credit chips," which seem to be similar to a credit card. Humans sometimes casually refer to credits as "credaroos," analogous to the way 20th and 21st century Americans would refer to dollars as "buckaroos."
Credits are the in-game currency used to purchase armor permutations in Halo: Reach. Credits are rewarded from gameplay actions, progression in Commendations, and completing challenges. They also function as Experience points by contributing to the player rank.
Halo: Spartan series
Credits are used in Halo: Spartan Assault and Halo: Spartan Strike to purchase additional power weapons, armor abilities and boosters. In Spartan Assault, players can choose to purchase these upgrades by spending XP (earned by playing the game) or by purchasing cR with real-world currency. In Spartan Strike, credits are earned in-game instead of XP, and real-world currency microtransactions are not present.
In Halo Infinite, Halo Credits (shortened as CR) are the in-game currency for purchasing in-game items at the shop, including the Battle Pass and Shop Bundles, both of which may include consumables like the Double XP Booster and Challenge Swaps, armor customization, armor coatings, and weapon coatings. These are purchased with real-world currency.