Understanding an FFmpeg command (blurring a video)
The above command line was copied from FFmpeg - Apply blur over face which also provides a good explanation:


  Note: The x and y offset numbers in overlay (60 and 30 in this
  example) must match the crop offsets.
  
  What this example does:
  
  
  Crop the copy to be the size of the area to be blurred. In this example: a 200x200 pixel box that is 60 pixels to the right (x axis)
  and 30 pixels down (y axis) from the top left corner.
  Blur the cropped area.
  Overlay the blurred area using the same x and y parameters from the crop filter.
  


So: the 200:200 is the size of the blurred area and 60:30 its position relative to the top-left corner. The input v is split into two streams v0 and v1 where v0 is blurred and the result is named fg. Streams v1 and fg are composed to the final stream v.