Inventory
Right next to the Daily Log in button is the Inventory button. It has a crate as an icon and this is where various rewards and / or items go. You obtain these from the Landing Zone (A Helipad where supplies are brought to you at a set time),
Challenge Mode, completing Events at the Event Board. These items may range anywhere from extra resources like Gold, Oil, Celerium, Diamonds, Boosts to shorter production times at respective Buildings, Rapid functions to shave of minutes or hours from research or training.
Tip: It may be a good idea to not use up all the resources that are stockpiled here. Just use them when you are lacking that much resources and can no longer go on another siege or wait for your resource generating structures to produce.