Rarity (officially known as ‘Weapon Quality’) usually signifies a skins rarity and value. The most common skins are assigned the Consumer rarity whereas the rarest skins are assigned the Covert rarity.
The only non-obtainable skin rarity is ‘Contraband’. This rarity has only ever been assigned to the M4A4 | Howl due to a DMCA notice relating to copyright infringement. This skin can now only be obtained through trading or the Steam Community Market.
* Does not include knives