From A Wiki of Ice and Fire
A thrall is an indentured servant in the Iron Islands. Thrall may only be obtained by paying the "iron price" (capturing them by raiding) under the Old Way. They are not slaves since they may not be sold and they may not be obtained by paying the "gold price." Their children may be free if they are drowned to the Drowned God. House Codd is descended from thralls and salt wives, and is looked down upon for it.