The shorthand "480p" refers to a 480-pixel vertical line that is progressively drawn together with other vertical lines. This mode is in contrast to 480i where each odd or even vertical line is drawn/interlaced at the same time. Interlacing requires less bandwidth, but results in a worse image.
The 480p resolution is considered a standard definition for televisions. This resolution is in contrast to high-definition televisions that typical start at 720p. Many televisions are now produced with an even higher resolution, often reaching a 1080p definition. This mode requires a lot of bandwidth--especially since television video often has a frame rate of 60 frames per second--but results in a superior image.