This primarily relates to standard-definition cameras. TVL is the number of Vertical lines a camera can record measured in a square equal to the vertical height of the picture. D1 is a 4:3 aspect ratio, so if the image was 3cm high it would be 4cm wide, but you only count vertical lines over 3cm. Given that D1 resolution is 576 (vertical) x 720 (horizontal) pixels that means anything over 576 TVL is in effect wasted. If you've done a bit of research already you would be forgiven for thinking it's the holy grail of a good CCTV system. It isn't.
Of far more importance is ensuring you use cameras with the right optical range for your particular application.
At D1 resolution your DVR records at around 400,000 pixels per frame. You need the lens to concentrate enough of those pixels onto the subject in order to capture detail. If a 550TVL camera allows you to read a number plate at 8 metres then using a 750 TVL camera with the same lens wouldn't be able to read it at any greater distance. But we are now entering into a grey area of CCTV lies and misunderstandings. Remember that true TVL is only measured over a square equal to the height of the image.
People selling CCTV cameras boast huge TVL numbers up to 1000TVL. Sadly they aren't telling you the truth, their 1000 TV lines are across the whole of a 16:9 aspect ratio widescreen image. To get the true TVL figure you would need to divide 1000 by 16 and then multiply by 9. They are actually only 560 TVL cameras.
Notwithstanding the fact that TVL isn't a measure of how much detail a camera captures the numbers used are often very misleading.
Upgrading to High definition 1080P provides a definite advantage due to there being 5 times the number of pixels compared to standard definition but you still need to think about what lens cameras you use and avoid wide-angle cameras which spread pixels too far apart as you move away from the camera.