From time to time you will find items designated as repurposed mods. These items are not common though and it may not be immediately obvious what you can use them for.
The mods can be applied to your gear (no Weapons) to improve their listed properties, but unfortunately not just any gear. Only items that have mod slots available in them and these are rare. As you rank up though and start finding better and higher level Gear you should start seeing more items with mod slots available. You can check for mod slots by scrolling down in the items information panel on the right. And If they are available then a 'Mod' option will be shown as available at the bottom of the screen (the same as with the weapons).
mods will apply a bonus to either one of the 3 skill settings (Firearms, Stamina, Electronics) or possibly add a bonus perk to the item such as increased XP. So it is worth finding and using these mods where you can.