I believe that
@lauravvv is correct. The music needs to have been released, originally, in the 50s, 60s or 70s. Remixes or covers that retain the original spirit and feel of the song are allowed - which is why others have said to expect a bunch of Glee covers.
Essentially, what the IDTC is doing is allowing for something like Guignard/Fabbri's original music cut from this past season where they used the Glee cover of "Holding Out for a Hero" which has a similar tempo and vibe as Bonnie Tyler's original version from the '80s.
I think it's also meant to limit the covers used from being distinctly different - ie Faith Hill's cover of the Janis Joplin classic "Take a Piece of My Heart" or Whitney Houston's "I Will Always Love You" which is very different from Dolly Parton's original version.