The Ones are part of a race called the Architects/Celestials.
They were an ancient civilization who were present long before the dawn of the galactic community and even the Rakatan Infinite Empire.
Think of them as Force-keepers/watchers, tasked with keeping the Force in Balance. Whereas the Force Priestesses were five beings that resided on the Wellspring of Life, the home of the midi-chlorians. These Priestesses were Force-ghosts. They were the watchers of the Wellspring of Life.
Most people here seem to think the Ones came first, and the reasons also seem pretty solid.
But I immediately thought, both in Canon and Legends that the Force Priestesses would have come first because the Ones aren't essential to the force. Yeah, the father is pretty much the force itself, but he dies, and the force still exists. I think, even in Canon, the Ones were never more than just REALLY powerful force wielders, but the Priestesses were essential to the living and unifying force.
But it would make sense that the Force priestesses would come about after the creation of LIFE, and thus the living force, whereas the Ones could have been around longer. Although, the Ones themselves are alive, so maybe the Priestesses would have had to be around before?