There are 2 camera scanning techniques- interlaced and progressive.

Interlaced Scanning divides the horizontal lines of the screen into 2 segments- odd number and even number, and refreshes the odd lines first even lines second in a continuous cycle such as 25 frames per second.  The time difference in updating the odd frame and the even frame would produce some distortion of the video image on the screen.

Progressive Scanning does not divide the screen into odd and even frames and it refreshes all horizontal lines of the screen one by one, that is, Line 1 to 2 to 3 and etc.  There is no flickering or distortion effect.

Most older models of cameras, presumably the analogue type, use Interlaced Scanning.