They are used to increase the energy tier of your armour, specifically used at tiers 8 and 9. On legendary armour tier 8 costs 1 prism and tier 9 costs 2, for exotic armour that increases to 2 and 3 prisms respectively. You can also spend 10 prisms at Banshee-44 to buy one Ascendant Shard which is used to fully masterwork (get tier 10) armour.