The Helots were Greeks that had been captured and enslaved by Spartans. The Helots performed many important duties in Sparta, including working on the land to produce food.
The Helots were incredibly important in the social structure of the Spartan society, because each Spartan householder needed to contribute a certain amount of food for the messes, and without the Helots tilling and taking care of the land, this could not be possible.
But the Helots were considered as the lowest class in the society and kept under constant watch.