Everyone playing seasons has to start a new character. You also don’t have access to your old stash / gear.

Especially for those who play with friends it can be really fun to put everyone at the exact same point again and allow you all to play through again. Seasons are usually big updates that add new unique / legendary items, new gameplay mechanics, new quests, etc. You play the season, get whatever loot and whatnot you can, and (if you want to) try to rise up the seasonal leaderboards. Once the season is over, you can move your character over to a “non-seasonal” character and continue to play them whenever. At this point, Diablo also merges those changes, new loot, etc. into the general game for everyone to get.

So you can generally get all the seasonal stuff later on even if you don’t play the season, but you have a better chance of getting seasonal loot during the season.

That’s basically it.