Think of your TV as being broken down into 1 pixel high rows (top to bottom). In a progressive, “p”, system, every time an image is drawn (scanned) on the screen, every single row is drawn. This results in whats called a frame. So in a progressive system, every image is a full frame.
In an interlaced, “i”, system, every time an image is drawn on the screen, every other rows are drawn. This creates what is so called a field. Two consecutive fields would make up a frame. Because of the speed at which the fields are played back (1/60th of a second), and the nature of the human eye, we perceive these fields as full frames, even though they are not. This is used for bandwidth reasons, because each time an image is drawn, only half the rows are drawn, it means half the resolution and half the bandwidth. This means broadcasters don’t have to spend money on new, more expensive, infrastructure to deliver 1080i to your homes, so they like it.
Progressive systems are “better” overall in that they will give a better image (either 720 or 1080). But because of the bandwidth issue, 1080i is the “better” choice for broadcasters for delivering 1080 content to homes. Eventually they will deliver 1080p, but just not quite yet.