‘And he looked, and behold, a well in the field. And lo, three flocks of sheep lying there by it. For from that well they watered the flocks, and the stone on the well's mouth was great. And to that place all the flocks were gathered, and they rolled the stone from the well's mouth and watered the sheep, and put the stone again on the well's mouth in its place.'
It would appear that the stone was so large that it was not easy to move. So every day the various flocks would gather at the well (water-source), waiting until all were gathered, and then the stone guarding the well would be removed and all the flocks would water there. It was possibly a private cystern owned by a group, with restricted access.