Seasons are just a refresh, where you need to create a new character that won't have access to any non-season items in order to participate. You won't be able to access your existing stash, and many unlocks will be reset. Usually there are new items/mechanics/events to go with a new season.

Blizzard haven't given us all the details on what Seasons will look like in D4, but at the very least it'll be an opportunity for the community to all begin "fresh" and enjoy the new content together. I believe they did say campaign progress will carry over though, and you won't have to repeat it with a seasonal character if you've already completed it before.

Once the season is over, your seasonal characters/items/unlocks will merge with your non-seasonal stuff. That way, you never lose any of that "progress", but you still get to enjoy the fresh grind each season. Plus, if you're competitive, the ladder provides an opportunity to compare your progress and achievements against other players. Things like who reaches level 100 first, dungeon completion times/difficulty, and lots of other things will all be tracked.