When a server closes or restarts for maintenence, this is where you'll find yourself. The Hub can be thought of as an adjoining server acting as a gateway to all others, a function evidenced by its most prominent feature -- teleport signs strewn around the lobby area as pictured. Simply right-click a sign and your game will attempt to connect to the chosen server, after which you'll appear either in your previous location on the server, or at the server's spawn area if joining for the first time. If you're having issues connecting, see the support section.
The Hub is also designed to entertain players while their server comes back up. You can explore, fight monters in the arena, or complete the three challenges -- maze, parkour and dropper. The entrance to the challenges is a
/server hub /server <name>