Strange Coins are an uncommon currency, notably used to purchase Exotic items and various wares from Xûr.
Strange Coins in The Taken King
Starting in The Taken King, Strange Coins became more difficult to acquire intentionally. Bungie removed the Weekly Heroic Strike, which had allowed Guardians to earn 39x Strange Coins each week.
Instead, Strange Coins are dropped more freely, for things like completing activities and turning in Bounties.