The screen resolution (screen frequency) is the distance of the individual screen dots and is indicated in lines per cm or lines per inch. The screen frequency is the reciprocal value of the screen resolution. If you refer to a 54 screen, 54 screen dots are positioned on a segment of one centimetre. The screen resolution is selected depending on the printing process and the surface structure of the printing substrate. In general, the finer the surface, also the finer the screen resolution.
Images below: screen resolution 7 lines/cm, 18 lines/inch