Watch some streams on twitch would help with that.
But in short, "You are a Pirate", you get jobs from the gold horders to find treasure chests, bounties on other (NPC) pirates, capture and transport livestock. You do this by sailing a ship (solo, with 2, or with 4 players) on two different size ships. Finding treasure is as simple as following the X on a map that you get, or it is sometimes a multi-part riddle (my favorites). Returning the treasures gets you gold, which you can then use to buy items, or customize yourself or your ship.
I have had some EPIC times in ship to ship battles, or in some cases running from or chasing another ship.