From Halopedia, the Halo wiki
The Battle Pass is a progression system for unlocking player customization in Halo: The Master Chief Collection and Halo Infinite. Every season will introduce a new Battle Pass, which contains different options for player customizations.
Halo: The Master Chief Collection
In Halo: The Master Chief Collection, the different tiers of the Battle Pass can be unlocked through Season points, which could only be earned by completing in-game Challenges and by progressing through the first 100 ranks in the game's rank scheme. Season points are a purely in-game currency, and cannot be bought through real-world purchases.
Please see the respective Season pages for their Battle Passes.
In Halo Infinite, every season will be introduce a new Battle Pass, but previous Battle Passes will remain available for purchase, regardless of the current season. Each Battle Pass will include a track for free rewards and a premium track, that latter of which requires purchase to unlock rewards. Players can select an active Battle Pass to make progress and unlock the tiered rewards, all of which will be exclusive to the Battle Pass and unavailable to obtain through other means.