They, i.e. the people belonging to that place, watered; or, the flocks were watered; it is an impersonal speech. A great stone was upon the well's mouth, to preserve the water, which was scarce in those parts, and to keep it pure.
Related Commentaries of Genesis 29:2
Genesis 29:2
2 And he looked, and behold a well in the field, and, lo, there were three flocks of sheep lying by it; for out of that well they watered the flocks: and a great stone was upon the well's mouth.