The video above is the Halo 5: Guardians Special Weapon Variants Locations Guide and shows how to unlock NAME the achievement featured in Halo 5 Guardians.
Throughout the campaign of Halo 5: Guardians you can find weapon variants for some of the usual weapons that are available for the player.
These special weapons are very rare, but more powerful and they usually have some functional modifications and a new skin. However, they do not share ammo with other weapons, so as soon as their ammo depletes they will be useless. Also, the secret weapons cannot be carried between levels.