From Halopedia, the Halo wiki
Seasons are a feature of Halo: Reach, Halo 5: Guardians, Halo Wars 2, Halo: The Master Chief Collection, and Halo Infinite, primarily focused on their games' respective competitive modes. Seasons have typically accompanied new ranked settings, a handful of title updates and patches and accompanied the release of DLC. Leaderboards on Halo Waypoint are additionally organised by season, with a new season seeing the reset of leaderboards and new matchmaking playlists.
Seasons were introduced to Halo: The Master Chief Collection following the 2019 release of Halo: Reach and the collection for PC, serving as the game's ongoing post-launch support with each new season bringing new cosmetic customization unlocks and gameplay improvements.
Halo 5: Guardians
Halo Wars 2
Seasons are present in Halo Wars 2. During the first year of the game's life cycle, seasons were released roughly once a month to coincide with downloadable content releases or other events such as the launch of Xbox One X. Since then, the output of Seasons has slowed to as little as one per-year. Seasons are a fixture of the game's ranked multiplayer, used for leaderboard and rank resets. Season refreshes are typically accompanied by a Title Update, and may have several mid-season patches.
Halo: The Master Chief Collection
Seasons were first introduced to Halo: The Master Chief Collection with the addition of Halo: Reach to the collection in December 2019. As of December 2020, 343 Industries intends to support the game with seasonal content roughly every two months, with current plans giving the game ten total seasons. Following the release of Season 10 will determine whether more seasons will be released. Even after the end of a season and the start of another, season rewards can still be purchased.
Season points are a new in-game currency introduced with the Season system. They can be used within the various unlock tiers of each season to unlock armor permutations, skins, nameplates, Firefight voices, and more. Initially, season points could only be earned by progressing in the game's rank scheme through regular multiplayer, though the addition of Challenges has resulted in a wider array of season point earning potential across multiplayer, campaign, Firefight, Forge and custom games. Season points are a purely in-game currency, and cannot be bought through real-world purchases.
Seasonal content for Halo Infinite multiplayer will include armor permutations. Two seasons have been mentioned for the game, though due to its pre-release nature these aspects are subject to change.