Simply put it, it is high resolution. There is analog and digital SD and analog and digital HD. HD naturally has an advantage. The new formats, SD, or HD, are all digital. Progressive is, for the same pixel count, better than interlaced.

SD is basically 720x480 pixels for NTSC and 720x576 pixels for PAL.

Basic HD forms:

720/60p with 1280x720 pixels. DVCPRO HD cameras record 960x720 pixels. HDV cameras record 1280x720 pixels.

1080/60i with 1920x1080 pixels. HDCAM and HDV cameras record 1440x1080 pixels, DVCPRO cameras record 1280x1080 pixels.

1080/24-60p with 1920x1080 pixels. HDCAM records 1440x1080 pixels. HDCAM SR records 1920x1080 pixels.

There is more to it, but if the camera head provides sufficient resolution, recorder section can record these pixel counts of the image and the lines of resolution are fully related to the pixel count. If 480p camera resolves 500 vertical lines, 1080p camera, that is equally optimized for its format should resolve 1,125 vertical lines. That is high definition, simplified.

If 480p camera resolves 500 vertical lines
How would you get more lines than the theoretical maximum? I'm confused here.