This can mean a couple things.
1) Incorrect DVD-key-
EVERYSINGLE Xbox360 motherboard is "married" to it's dvd drive with a unique dvd-key. If an idiot attempted to mod the console, or "repaired" it by putting in a new dvd drive. The key is lost forever, and nobody other than Microsoft can fix it. (It'll cost you like $30 more, just to buy a new 360 Vs paying them to repair it)
2) The laser is dead/dying
The optical laser in the dvd drive needs to be replaced, (or tweaked for a temporary fix) A brand new laser costs $8-$13 off of eBay with free shipping included. Email me at [email protected]
for more detailed instructions or part numbers.