What does 0 mean in YUV420
As explained on the Wikipedia Page on Croma Subsampling:


  The subsampling scheme is commonly expressed as a three part ratio ''J'':''a'':''b'' (e.g. 4:2:2) or four parts if alpha channel is present (e.g. 4:2:2:4), that describe the number of luminance and chrominance samples in a conceptual region that is ''J'' pixels wide, and 2 pixels high. The parts are (in their respective order):
  
  
  ''J'': horizontal sampling reference (width of the conceptual region). Usually, 4.
  ''a'': number of chrominance samples (Cr, Cb) in the first row of ''J'' pixels.
  ''b'': number of changes of chrominance samples (Cr, Cb) between first and second row of ''J'' pixels.
  
  
  …


In other words, your 0 says there's no vertical chroma subsampling at all. Instead, you just assume that columns within the four pixel wide subsampling region have identical chroma.