Weapon Mods can be purchased and found in Remnant: From the Ashes and they give your weapons powerful buffs and alternate attacks. To use weapon mods in Remnant, you’ll need to have a weapon mod equipped to your gun.
You can do this in the Character Tab of the pause menu. Highlight the weapon you would like to equip the mod and then press the X Key on PC or (X Button Xbox, Square Button PS4). Doing this will open your available mods. Highlight the mod you want on the weapon and then confirm to slot it.
Different weapon mods do different things. Some give you passive abilities like resistance to certain types of attacks, while others will give your weapons powerful attacks or other effects must be activated manually.
When you have a weapon mod that must be activated manually there will be a charge associated with it. When fighting look to the bottom right corner of the screen and look for the white bar around the weapon mod. Once this bar is full you can activate the weapon mod by pressing the RB/R1 Button or F Key.
Mods will be earned throughout the game and some can purchased as well. The female NPC vendor in the armory area will sell mods and you can craft them.