I think it's a carriage gear hookup? I don't know how to describe it really, but it's the plastic piece that connects via 2 screws the laser assembly so it can seek on the disc, and it is moved by the (worm?) gear. Funnily enough everyone talks about the centre gear failing, but mine is fine, it's this other piece that's had 2 teeth snap off so the (worm) gear just spins freely and the laser won't move at all.
Here's a pic:
Is it possible to get a replacement plastic piece?