From a lore perspective, the elemental plane of fire, and the home of Ragnaros, the Firelord, ever since the Titans imprisoned him and his followers there.
From a gameplay perspective, it’s the middle raid tier of Cataclysm, an instance with seven bosses- Beth’tilac, Lord Rhyolith, Alysrazor, Shannox, Baleroc, Majordomo Staghelm and Ragnaros himself. It’s also involved in the Dragonwrath legendary questline, since many of the quests involve killing bosses there.