Ultra Hardcore (UHC) is a Minecraft mode that removes automatic healing, so you can only recover with Golden Apples. On this server, even that gets a twist, since Golden Apples are crafted with golden ingots instead of nuggets.
After you connect, you've got two ways to play. UHC rounds start once 4+ players sign up with /play, and when the match begins it's all about surviving, gearing up, and being the last one standing. Mine, loot, fight, or hide, whatever gets you through the round.
If there aren't enough players to start a game right away, you can still get some practice in the survival world with /survival, then head back to the lobby with /lobby when you're done. Good place to warm up before the real fights.