A Loadout or Loadout Slot, is where you can save a primary and secondary weapon configuration or Premade so that they are available to you during a match. All agents start the game with two Loadouts that contain the default Assault Rifle and Light Pistol.
To be able to store additional weapon configurations, you can purchase additional permanent Loadout Slots for 1000 GP or 250 ZCoin. It is possible to have up to 5 Loadouts. Before the Parity Patch there was a sixth Loadout Slot that was only used for Heroes to store their custom weapons.
While at the Loadout Menu the starting Loadout Slot can be changed by double-clicking on the Loadout number near the top of the menu. When you next respawn you will use that Loadout. In the PC version of Blacklight: Retribution you can use the mouse wheel to select a different Loadout while dead or select a Loadout by clicking on it.