As a character earns experience points, it also gains advancement points (AP). These can be spent on advances. Most advances require a minimum rank, and many require other advances as prerequisites.
Advances give you new options in building class load outs.
Most advances are wargear: armor upgrades, trinkets, grenades, health packs and so on. Weapons and weapon mods are purchased with requisition points, either directly or through a supply drop.
Weapon mastery is another type of advancement. This will reduce the loadout point cost of a particular weapon.
At rank five, you can unlock elite classes.
Each faction has its own advances.