Strange Coins are a form of currency used to purchase items from Xûr during his visits on the weekends within Destiny.
In Destiny, Strange Coins can be randomly awarded for completing Public Events with a Gold Rating, completing Crucible matches, completing Weekly Nightfall Strikes, decrypting encrypted engrams, opening loot chests in the Prison of Elders, opening reputation packages, and turning in bounties.
Strange Coins returned in the 30th Anniversary Pack for Destiny 2 where it was used to purchase bounties from Starhorse in Xûr's Treasure Hoard. It was eventually used by Xûr once again as currency to purchase his wares from the Tower Bazaar beginning in The Final Shape. Strange Coins are earned from playing ritual activities and from the Season Pass.