Hi,
In MPEG2 the Picture is divided into slices which inturn into MacroBlocks which inturn into Blocks.
For a 4:2:0 (yuv) format encoding, each macroblock contains 4 blocks of Y (luminance) 1 block of Cr and 1 block of Cb(Chrominance) thus 6 Blocks.
The Coded Block pattern specifies that whether that particular block in the Macroblock is coded (DCT + quantisation) or not i.e if the cbp for that block is 1 then it is coded and is present in bitstream.
Hope this was useful.
Regards,
KJN