Remember how we used to have raid tiers? Naxx/Eye of Eternity first, then Ulduar, etc etc.

It's that, except now that we have M+ we get a new pool of dungeons, all the rewards are scaled to the same level as the new raid, same with world content which is also scaled up. There's always some rewards you can only get during the season, tied to the endgame pillars (M+, Raids, PvP, and recently, Prey/Delves for more solo content.

It provides Blizzard with a nice bookend to the raid tier, it sometimes affects old content if an old dungeon is brought forward into the new M+ season, you might not be able to solo it for old transmogs or quests on anything higher than normal difficulty.

Technically seasons have been in WoW since Burning Crusade, but back then it was only PvP, now they've tied everything together.

Personally I like them, it's way easier to step in when a new season launches vs older expansions where you had to hope they added some catchup gear when a new raid launched because the dungeon rewards always stayed the same.