Sounds like you should ask this in the Storage forum.
Anyway...
Assuming you refer to a cd-burner, the buffer is a temporary storage for the data that is being sent to your cd during the writing process. The larger it is, the better.
If you have buffer-underrun protection like Burnproof, this keeps the buffer from running "dry" or empty, so your cd-writer will only write when there is something in that buffer, otherwise it halts until the buffer refills again.
Very important if you have a slow CPU or a big workload going while burning.
Without this protection, you would end up with a coaster.
SCSI-burners are less prone to produce coasters, as they do not rely so much on CPU-availability.