Pergium is a radioactive element used for power in 23rd century nuclear reactors such as the PXK. Pergium is used by thousands of worlds as a source of energy. One key source of pergium is the planet Janus VI, where it is mined by a Federation colony in conjunction with the Horta. (TOS: "The Devil in the Dark")
There were several pergium mines in operation in the Sappora system in the 24th century. One was owned by Yanas Tigan and one by the Ferengi. The Orion Syndicate was also working its way into the industry in 2375. (DS9: "Prodigal Daughter")
It's primary use aboard 24th century starships was to regenerate the filters of starship environmental control system.
Captain Janeway sought a supply of them from Bahrat's station near the Nekrit Expanse in 2373 to regenerate the ships filters. He said that it was a rare commodity in their region, but there were some suppliers he knew. Later, Wixiban told Neelix about a way to acquire some, trading medical supplies. Initially promising Commander Chakotay 45 kilograms, they only ended up returning with 20. (VOY: "Fair Trade")