Perhaps the initiated already know all about interlacing, but I only knew a little, and I wish I had known more.
Basically it harks back to the mechanics of the television.
Footage is 25 distinct frames per second (at least by UK frame rates). Each frame is a still image but when played together they give the illusion of motion, this much we all know.
There's a lot more to it though... do yourself a favour if you're thick like me, and read these: