The image for an MPEG compressed video is splitted in 16x16 blocks. For each frame (excluding the 1st of course), the compressing algorithm tries to to matchthat particular block with a block in the previous frame (it searches the previous frame to see where there are the fewest differences). If it can do it it only encodes the differences and the position in the previous frame; this is called an inter(predicted) block. If however it can't match that block with the previous frame then it needs to re-encode from scrach; that's the intra macroblock. As you can understand after a shot cut there will be much more intra macroblocks.

There are some more info in the wikipedia: https://en.wikipedia.org/wiki/Macroblock and also there's a nice article explaining some of the magics of H264 compression: https://sidbala.com/h-264-is-magic/