First of all, it's iMac, not IMAC. Mac is for Macintosh, it's not n acronym. (MAC = media access control)
Secondly, have you tried
Google? There are very many people who have experienced the same error. Here are some of the recommendations:
1. Bad DVDs-try a different brand
2. Laser Calibration Error, get a lens cleaner
3. Reboot
4. Switch burning speed form 8x to 4x
5. Force Quit the Finder, and relaunch it. then try to burn