A zephyrium bar is a bar that can be created by using zephyrium ore on a furnace, requiring level 50 Smithing and granting 38.5 Smithing experience. The zephyrium ore itself can be mined from the rocks scattered throughout the dungeon, obtained as a monster drop, or bought from the Smuggler in the starting room.
A zephyrium bar can be smithed on an anvil through the Smithing skill to create Zephyrium equipment, granting between 55 and 59.5 Smithing experience per bar used. A list of zephyrium items that can be smithed from zephyrium bars and the Smithing levels required can be found here.