A salt wife is essentially a bound concubine in ironborn culture. They are usually the women kidnapped during raids. An ironborn may keep several salt wives, but only one "rock wife", which is of ironborn origin.
Salt wives have a low status in ironborn society, similar to thralls. Salt wives don't toil in the fields and mines as thralls do, but they do serve as concubines in all but name.
In some ways salt wives are the mirror opposite of sorts to paramours in Dorne. Unlike salt wives, Dornish paramours are relationships based on love, and which are often marriages in all but name. Paramours are high-ranking and respected positions in Dorne, while in contrast, salt wives have a low-ranking social position similar to thralls. Children born to salt wives are not considered bastards, but they rank behind children of a lord's rock wife in the line of succession. Meanwhile, children born to Dornish paramours are still considered bastards.
In the books
A salt wife isn't quite the same thing as a slave, as even the ironborn raiders don't believe in "slavery" as such. The difference is that a salt wife cannot be bought and sold, or traded: an ironborn may only keep salt wives that he has personally captured in raids, as this is considered "paying the iron price". Also, the ironborn generally look down on selling slaves, as they still think of their captives as human beings, albeit on the bottom rung of society, and not animals to be sold.
Salt wives are a somewhat unique cultural institution to the ironborn: they are actually considered "married" to an ironborn lord, and are technically a step above actual "concubines" - though most are basically treated as little more than concubines. The ironborn don't technically consider this to be an instance of "polygamy" either - an ironborn can only have one rock wife.
The children of salt wives are specifically not considered to be bastards, and they use their father's surname - but they rank behind a lord's children by his rock wife in the line of succession. While not officially disparaged culturally, those descended from salt wives are still sometimes disparaged - much as the low-ranking fifth son of a lord isn't considered as prestigious as his eldest son and heir. After Dalton Greyjoy was killed at the end of the Dance of the Dragons, the Iron Islands were plunged into civil war, because he had never taken a rock wife and only had sons by his salt wives. Other Greyjoy cousins felt they had better claims as the sons of rock wives, and Dalton's sons also squabbled among themselves given that there was more than one "eldest son", one from each salt wife, each of whom felt they had a better claim.