The Old Ones were an authority to whom Korob and Sylvia owed some form of duty or fealty. Presumably, they sent the two explorers to Pyris VII to learn. Korob reminded Sylvia of this duty, but she had become intoxicated by sensation (something that did not exist where she and Korob were from), and rejected his opinion that her actions were not in accordance with her responsibilities.
The Old Ones made several mistakes. They gave Korob, the senior member of their exploration team, a more powerful transmuter, but failed to realize that he lacked the will to use that power, even when his colleague Sylvia became irrational. They failed to realize the dangers of being immersed in sensation (the cause of Sylvia's irrationality). (TOS: "Catspaw")
The Old Ones were presumably named after the Old Ones in H.P. Lovecraft's Cthulhu Mythos, for which Robert Bloch, writer of the episode, contributed several stories. Bloch also made mention of the "Old Ones" (albeit referring to a different species) in "What Are Little Girls Made Of?". (The Star Trek Compendium)