Armor mods allow you to add additional upgrades to your armor that can do things such as decreasing grenade or super cooldown. All mods are one-time use and the only way to remove it from a piece of gear is to destroy it. Mods can acquired from the Gunsmith using mod components that can be obtained from dismantling other mods or pieces of legendary or exotic gear.
Each piece of legendary or exotic armor has a single port called a mod slot. This slot is where you can install the armor mod of your choice. An important thing to note is that there is only one slot to install a mod.