480p means a standard definition video signal PROGRESSIVE SCAN. Normal video is 480 lines of resolution, and the standard video system here in the US uses INTERLACED scanning. Progressive scan just means the picture is drawn more times per second and results in a smoother image.
For your to see 480p, you need to ahve a TV that supports progressive scanning. This usually means having a HDTV set, although there were some older TVs that came out before HD that support Progressive scanning.