The aforementioned coins are obtained by turning in ore from The Rift, and are then traded on for various items. Note that the coins are not used to get class-specific sets -- these will come from the dropped gems, covered in a future guide. The coins can be traded for some useful consumables, or other general armor pieces (which still look like fairly decent items).
Almost all armor items will cost 2 coins, with the one exception being a 4-coin ring, and the consumables are between 1-2 coins each. It looks like even if you aren't lucky enough to get your hands on a gem during a raid, grabbing some ore to trade for coins can still net you some nice new shineys. The full list of coin loot, with stats and costs, is available via the link below.