Quite simply, it's a preset for the Perfect Viewmodel mod for Payday 2 that moves your weapons to the center of the screen. I personally find it really fun, especially with shotguns and pistols, so hopefully you'll enjoy it as well.
It should cover most (if not all) of the weapons in the game, as well as many custom weapons.
Important notice: some of the weapon mod mods, especially those that add vertical grips and underbarrel launchers, or in any other way modify the weapon animations, may result in the weapon not being moved to the center of the screen.
Additionally, sometimes after throwing a grenade the weapon will return to it's vanilla position - it will go back to the middle if you crouch or aim, it's an issue with PVM more than the preset.
To install, place the "mods" folder into your PAYDAY 2 directory. If you have PVM installed and have your own modifications, go to mods>>saves>>PerfectViewModel_Options.txt and back it up somewhere.
If I missed any weapons, simply adjust the setting yourself. To put the weapon exactly in the middle, take the steelsight's x value and copy it into standard and crouched. Adjust y and z to not clip into your face and it's done.