A moratorium is in effect on adding information from Fire and Blood until December 20th.
In Ironborn culture a salt wife is a concubine. They are the women kidnapped by the ironmen during raids. An ironborn may keep several salt wives, but only one ironborn "rock wife". Salt wives are bound to their captors in a religious ceremony, though it is considered a lesser ceremony than that of their rock wife. The number of salt wives kept by an ironman is a symbol of status and virility.
Unlike Dornish paramours, salt wives have a low status in the society of the Iron Islands, on the same level of thralls. They are not, however, considered whores or slaves, and their sons can even inherit property or lands in lieu of trueborn heirs born of a man's rock wife.