Items, including Weapons and Gear all have a rarity. The rarity of an item gives a rough idea how rare the item is, and for how much it will sell at a Faction.
The rarity of the item also determines what kind of icon it will have inside the Inventory, as well as other things items are inside of, such as the Stash.
There are six rarities. They are often also referred to by their colour by Prospectors.