- "We raised our Kings from our own ranks, and used beaten foes as thralls to work our mines and farm our land."
- ―Yara Greyjoy
A thrall is a bound servant in ironborn culture. They are people kidnapped during raids and forced to toil at tasks such as agriculture and mining, which the Old Way dictates the ironborn are not to perfrom themselves. In a typical raid, most captives are put to the sword, but those who are unlikely to cause trouble and/or have desirable skills are taken as thralls. Like "salt wives," thralls have a low status in ironborn society.
In the books
A thrall isn't quite the same as a slave, as even the ironborn don't believe in "slavery" as such. Thralls are specifcally captured in raids, as this is considered "paying the iron price", and are not allowed be bought or sold. In fact, 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. Thralls are more like "prisoners of war" and not considered "property" in any sense, unlike slaves. Thralls can possess wealth and property in their own right, and any infant born to two thralls on the Iron Islands is considered ironborn, as long as they are baptized in the faith of the Drowned God. Such children also cannot legally be taken away from their parents until the age of seven, the age at which most would first join a ship's crew or begin an apprenticeship. There are even Houses that descend from thralls, such as House Codd and House Humble, though they are looked down upon because of their origins.
According to sketchy accounts and oral tradition, the First Men apparently practiced thralldom when they first migrated to Westeros about 12,000 years ago. Over the centuries, however, it fell out of practice, and had ceased many centuries before the Andal Invasion (6,000 years ago), and possibly even before The Long Night (8,000 years ago). Maesters have taken this as a strong piece of evidence (one of several) that the original ironborn were indeed an offshoot of the First Men, not - as a few of the ironborn claim - that they were an entirely separate ethnic group from the First Men or Andals, and came to the islands from far across the sea.
The soil of the Iron Islands is very poor, and many minor lords cannot afford draft animals like horses or oxen; thus their thralls and smallfolk have to pull the plows themselves. The only appreciable natural resource the Iron Islands possess is a wealth of mines yielding iron and other metal ores: their yields are typically produced from backbreaking mining by thralls. Only the largest island, Great Wyk, possesses plentiful enough iron mines that its ruling Houses gain more wealth from the land than from the sea. Thralls who work the fields of the Iron Islands usually grow old and can even raise their own families, but thralls doomed to the mines often die from exhaustion after only a few short years.
Aegon I Targaryen put an end to the ironborn taking thralls from Westeros itself, but it is tacitly acknowledged that raiders still take thralls from further afield, i.e. by raiding the ships and coasts of the Free Cities and Slaver's Bay, or by periodically conducting small raids on the fishing villages of the Stony Shore or Bear Island.