Are you talking about the install disk that came with machine?
If the Mac shipped with Snow Leopard, 10.6.x or older, the gray disks which came with that Mac are what you use to erase the drive and reinstall the OS. Make sure to include the bundled gray disks when you sell it.
For Macs which shipped with Lion or later, there are no disks. See what to do next in such a case.
1) If your Mac has the same version of OS X installed it shipped with and is currently on the drive, restart and hold down Command+R to boot into Recovery Mode. Use Disk Utility to erase the drive, then reinstall the OS.
2) If you upgraded to a newer OS than what shipped with the Mac, restart and hold down the Command+Option+R keys to boot into Internet Recovery Mode. Use Disk Utility to repartition the drive. You must do this to remove the newer hidden recovery partition. Otherwise, if you boot into the normal Recovery Mode, it will want to install (example) Mavericks, if that was the upgraded OS. Since the Mavericks install is tied to the App Store account which installed it, the new owner will not be able to reinstall the OS when using Recovery Mode. You must return the Mac to the version of OS X it shipped with. Repartitioning will wipe out the hidden recovery partition. Then, while still in Internet Recovery Mode, reinstall the OS. The version of OS X the Mac shipped with will be reinstalled, and a new recovery partition will be created.