Skyblock servers are multiplayer versions of the original sykblock survival challenge map. You begin on small island with limited resources and the goal is to expand the island until you have everything needed to survive. Because of the limited resources players must build cobblestone generators and renewable resources, like tree and crop farming to expand the island.
Typically the servers like the ones listed here have economy features that allow players to trade. Leaderboards for competitive players, and there are also variations of Skyblock servers, such as Skywars that combine Skyblock and PvP style gamemodes.