Progressive or progressive scan is basically an opposite to interlaced picture.
In standard, interlaced analog TV, the picture on TV is changed 50 or 60 times (50Hz in Europe/PAL systems and 60Hz in most NTSC systems, including the American one), but the picture contains only every other horizontal line and the line between is left "empty" -- and the next frame then contains only the horizontal lines missed in the last frame. So, therefor in interlaced picture, at 50Hz frequency, the picture actually changes only 25 times per second (so its framerate is 25fps).
But in progressive scan technology, every picture contains everything, so therefor 50Hz progressive scan video changes the picture 50 times per second, having the framerate of 50fps.
