a. There is something faulty with the disc.
b. There is something faulty with the optical drive.
It's up to you to test and determine which is the problem. All we can do from here is tell you that it is one of those two things.
Since you didn't specifically mention it, I have to ask: is this a Blu-Ray disc? Because if it is, forget it. The optical drives in Macs don't read those, and Mac OS X doesn't offer any built-in support for Blu-Ray.