The Tenno's weapons function properly on their own, but to reach the pinnacle of power, one must add the correct mods.
These mods drop randomly from any enemy, and can be anything from a power mod(which allows a Warframe to perform special skills) to granting additional health, shields, bullets or damage. They are essential to the Tenno arsenal - and so is upgrading them to make them stronger.
Each Warframe has polarities: all have 4 Ability slots, two more slots that vary between each frame, and an Aura polarity. Weapons may or may not have polarities.
By default, Frames and Weapons have 30 energy for mods at lvl 30; 60 energy if installed with an Orokin Reactor and Catalyst, respectively. Installing a mod with the same polarity halves energy cost; any other polarity and the cost is 30% greater; slots with no polarity has no effect on energy costs.
Finally, Auras give passive effects for the entire squad, ranging from energy regeneration to additional weapon damage. Aside from Excalibur, all frames have a polarity in their Aura slots; unlike normal mods, Auras give energy. Matching polarities gives twice as much energy as stated on the Aura; on the flipside, it gives 30% less energy when the polarities don't match.