Indentured servitude was the practice of an individual being forced to perform work for another for a fixed period of time. As such, it was a form of slavery.
The Ferengi Commerce Authority had the authority to punish Ferengi females who earned profit, an illegal activity for them, by selling them into indentured servitude. Liquidator Brunt threatened to do that to Ishka if Quark did not get her to confess to her having earned a profit in 2371.
When Quark warned his mother about the possibility, Ishka said she'd rather be sold into servitude than sign the confession, which to her was the same as admitting that her actions were wrong. (DS9: "Family Business")