Welcome to the forum. We are happy to have you as a fellow DVDFab user. The Quality percentage is a relative indication of the amount of compression to be applied to the selected titles. The lower the number the more compression and generally the lower or less satisfying the output video quality, but this is subjective and also depends on the bit rate of the original DVD. To anticipate your next question, I have had good results as low as 65% with some movies, but prefer to keep the number higher, above 80% if possible.