Resources include a variety of items, such as Energy Packs, which can allow you to fight in more battles or boost your team's statistics temporarily. Perform research to unlock new Resources. While most require Gold to purchase, Silver is also needed for getting Resources for your Avengers.
In the Resources menu you can see a variety if information - what is required to get the Resource, and exactly what its statistics are for your team. You should definitely look at purchasing many resources to help you through the game.
These function similarly to certain resources, in that they boost your Avengers' statistics. You can purchase new Uniforms using Silver, Gold and S.H.I.E.L.D. Points. Like Resources, you may need to do a certain amount of research before you are given the option to purchase a particular Uniform.
You can use Uniforms to change the class of your agent by utilizing the Uniforms menu.
Supplies are useful in combat, especially for rescuring yourself from a difficult situation. If you select the 'Inventory' icon, which looks like a locked chest to the right of your weapons, you can pick from Healing, Offense or Defense. You can also see all of your supplies in one list.
Use the Gear menu in the Purchase window to kit your team out before heading into combat. This will allow you to buy new Healing, Offensive or Defensive items.
Distress Calls are immensely useful tools that can really get you out of a tough spot when in combat. If you are struggling in battle, you can let out a Distress Call to get help from an ally or Hero. You can only use one Distress Call per battle.
How to get Distress Calls:
The only way to 'earn' Distress Calls is to visit your Friends'/Allies' maps.