Does it have a slightly sticky side (and probably black) and about 1/2 mm thick? It could be the soft plastic ring that cushions the drive motor to the bottom of the disc.
If that has fallen off then its more than likely your discs will slip when being spun up making a horrible noise and scratching the center hub of the disc.
Its possible to refit it; but to be honest, if you knew how to do that you probably wouldn't need to be asking what it is...
..or it could just be a bit of damping felt that probably wasn't doing anything useful.