season is like playing on a private server with everyone else who’s playing seasonal. you can carry none of your non-seasonal stuff in. you have a new stash in there and are paragon 0. there are a whole new set of achievements to go for, and a lot of them have rewards that are instantly given to you when you get the achievement. the big sell is that you can wind up with a 6pc set within a few hours of starting out on a new season by doing the first few lists of achievements. If you finish all of them before the season ends you permanently get another stash tab for nonseasonal and for all future seasons.
at the end of the season anything seasonal becomes nonseasonal and your paragon experience from seasonal get added to your nonseasonal pile
its basically a fun excuse to start absolutely fresh at the same time as other people and i think usually there’s more people playing seasonal than non, while one’s running