Glimmer (1) is the most common currency. You will receive Glimmer for just next to every activity in the game, including: performing various tasks, killing enemies and dismantling: weapons and armour, mods, recolours. Do not feel obliged to save it - in the final phases of the game you will not find many ways of using it. Legendary Shards (2) are gained by dismantling legendary or exotic class weapons and armour (purple or gold background). It serves a number of purposes: it is spent while using mods, doing infusions and in the case of special purchases (e.g. from merchant Xur). The last currency - Bright Dust (3) - serves only one purpose: purchases from Tess Everis. The items you can purchase from Tess are not of any priority, since they are mainly cosmetic, she sells recolours and skins for vehicles or ghosts. You will receive Bright Dust only by dismantling items found in Bright Engrams (which you receive for "levelling up" after you reach level 20 - the bar is filled, but the level will never actually rise above 20).