Interlaced video is a method to display an image progressively. Images are drawn line by line, and interlacing means that the odd lines are drawn first, and the even lines are "filled in" later. This effectively doubles the resolution of the image. The two parts of the image are called fields, and are transmitted one after the other.
Telefunken invented the method in the 1920s. Today it is widely used for displaying images and video.