Meta Knowledge refers to a player possessing information that a Player Character would not or should not know. In the context of Path To Power this mainly refers to information about the world setting from the Moonless Age comic that a Tei'kaliath would not have access to.
An example of Meta Knowledge is the true nature of the Vel'Sharen clan, such as the fact that Khaless is actually a sentient demon, which is known to readers of the comic but would be unknown to a player character.
It is generally advised that players avoid using Meta Knowledge, though it can be hard to divorce what a character knows from what their player knows.