Mods are special attachments or alterations that can be added to weapons, armor, and Power Armor. You can purchase mods from some traders, and you’ll find a few loose mods lying around the Commonwealth, but almost any mod can be hand-crafted or plucked from existing gear.
Your Pip-Boy’s Mods tab displays all of the loose mods that are currently in your inventory. In most cases, a mod’s name indicates the items with which it can be used. A pipe bolt-action suppressor can be attached to any pipe-bolt-action gun. A shadowed leather arm mod can be placed on any leather armor arm piece. The Mod tab also displays the weight and value of each loose mod. Most of these items are relatively light, but a single crafting session can result in any number of loose mods; store or sell superfluous mods to keep them from weighing you down.
To attach an existing mod to a piece of gear, you simply need access to the appropriate workstation. With the required components and perks, you can build nearly any mod you’ll find in the Commonwealth.