Membership is a unique status assigned to players who purchase access at the online shop. They receive an item from an admin after purchase, which can be used to grant them the allotted membership time they paid for. Members also get a bright blue name in-game and in the Discord. There are better prices for longer memberships, including a free World Blessing with some purchases (also sold separately in the store).
However, membership is not mandatory to fully experience the game. All resources and monsters are available outside of these members-only areas in some manner.