The way of putting a video (moving image) picture on a screen, comes from the 1920's when the first CRT (cathode ray tube) TV's were invented. Basically, the video image is drawn on the screen as a series of lines from the top to the bottom of the screen. Due to the low bandwidth and resolution available, each image was broken into odd and even lines and the TV would first display the odds then the evens. This is called interlaced scanning and is indicated with an "i" after the number of lines of a video specification. E.g. 480i . The number (480) corresponds to the amount of lines there are in the complete video picture. This line resolution can vary depending on the source used, with a VCR (video machine) being the worst.
In later years, it has become possible to store complete images on DVD and therefore it was not necessary to break the images up into halves. These images were de-interlaced, or differently put, all of the lines of a video picture were drawn in one go from the top to the bottom of the picture. This is called progressive scanning and is denoted by a "p" after the number of lines. E.g. 480p .