Essentially, TVL or TV lines are how resolution was measured before digital video recorders appeared on the market. Nowadays cameras and older monitors use TV lines term to refer to the quality of image you get.
TVL determines the resolution quality of analog security cameras or TV monitors. Basically, you should know that the more TV lines a camera has, the better the image quality will be. However, for spy cameras, depending on the lens, it also means the narrower view angle.
TVL consist of dark and bright lines. 520 TV lines means that there are 260 bright lines and 260 dark lines. For example PAL systems use 625 TV lines, while NTSC uses 525 lines.