A bad burn is when the DVD you copied does not playback properly in either your PC or stand alone DVD player. Could be skipping, freezing, or the disk not being recognized. Most common ways to prevent is to use quality media, don't try to burn at high speeds, and make sure the media you are using is recognized by your stand alone player. Most DVD players will play the DVD- media, newer ones will play DVD+, and many newer ones will play both, as well as DVD R\W.
Another thing to consider is recently ther seems to be an increase of defective brand new DVD movies. The original plays fine in your PC and stand alone, but when you try to copy it won't because there are errors on the disk and can't be read. When you receive one of these don't spend a great deal of time trying to copy, it just won't happen. Just return to place of purchase and get a new one, you'll be happy you did and spend that wasted time doing things more productive.