The Kaerls are the warrior serfs of the Space Wolves and Planetary Defense Force of the homeworld of Fenris. They are under the direct command of Space Wolves and are known to be fierce, effective fighters.[1c]
Kaerls are armoured in Carapace armour and wield heavy Skjoldtar autoguns, known to be more powerful than the autoguns and lasguns of the Imperial Guard. When aboard void ships, they are equipped with blast-armour with translucent face masks.[1a][1c][1d][1e]
The term is maybe related to the scandinavian "Karl" or german "Kerl". While "Kerl" is a general term for guy, "Karl" referred specifically to able-bodied men who could be conscripted for an army.
See also Housecarl.