An indentured servant was, simply, a slave
Although it was considered voluntary term servitude (typically believed to be of seven year's duration), many indentured servants were sold into servitude by their families. These servants were poorly treated and frequently did not receive anything more substantial for their years of labor than a ticket to the New World. Frequently, the contract was part of a master/apprentice relationship, ensuring that the master would teach and the spprentice would not be able to run off until they were fully trained. Decide two years into it that you don't really want to be a blacksmith, you'd rather study apothecary? Too bad, Chuck, not your decision.
The term indentured servant comes from the paper their contract was written on. It was torn, notched, given teeth, so the two halves of the document would only match each other, similar to the way you would tear the claim check in a pigeon drop.