Can someone explain "seasons" to me?

[Wrath of Foamy]

Not if you just recently beat it during the current season. You can carry the character over that beat the campaign. The seasonal content unlocks when you beat it, but you can still continue with the same character.

When the new season starts in May, that's when you'll have to start anew.

I played Eternal with my Druid.

I found the idea of seasonal v eternal to be a bit confusing.