In DVDFab, the checkbox forces deinterlacing to occur, bypassing the auto-deinterlacing logic in the program. You should not need to check it unless the DVD you are converting was not authored correctly. The symptoms can vary, but there is often a jerky look to the converted video, sometimes with jagged edges on fast moving objects of high contrast. Interlacing is the process of splitting a frame of video into two fields, each (for NTSC) containing 262.5 of the 525 horizontal lines that make up the analog video signal. It allows analog TV broadcasts to take up less bandwidth than would be required for a 525-line progressive scan (non-interlaced) picture (since only half the picture is sent at once), but (theoretically) it looks the same.