This is what I wrote about interlacing

TV screens work by drawing every odd line of an image, then every even line, then every odd line, and so on. As a simple example, consider a simple 3-frame animation showing a face moving across the square. What you want is something like this:

But what your TV shows you is something more like this:

Every other line, you see. The awkward thing is, of course, that superimposing one image on the other doesn't give you the proper image, so your computer has to do some clever things behind the scenes, to turn this:

into this: