From Destiny 2 Wiki
Rare Reef coins struck during the height of the Golden Age. Of great value to certain collectors.
Silver is an Eververse currency in Destiny 2.
Unlike Bright Dust, which is an another Eververse currency, Silver can be obtained only by paying for it with actual money.
Silver can be bought on Destiny 2's store page for the respective platform. It is sold in packs with different quantity of Silver and cost.
There are five packs in total:
Silver is used to buy cosmetic items and item bundles at Eververse. Bundle price is reduced for each item the player already owns.