The inventory is where you are going to hold non armor items such as currecty types Glimmer and marks to consumable items that you can activate for a battlefield boost and and materials necessary for upgrading your armor and weapons as you progress through the entire missions in the game.
Currency Glimmer:
Destiny Glimmer is a programmable matter that was once used as a source of power during the Golden Age, and is now the main currency of The City. Glimmer is used to purchase many items and services from vendors in The Tower.
Players can earn Glimmer in many ways, including for every enemy killed in all game modes, for each completed mission and PvP win, dismantling weapons and equipment, and for exchanging items found in the field with vendors.
Glimmer can also be found in various chests hidden across the map in caves and rooms.
Currency Vanguard Marks:
Vanguard Marks are currency among the Vanguard, rewarded to Guardians for the completion of high-level Strikes, Patrols, and Vanguard bounties. Vanguard Marks can purchase powerful armor and weapons.
Reputation has to do a lot with it, if you have enough marks does not mean ou can buy them without the required reputation level.