A macroblock is a portion of image that consists of 16x16 picture elements (pixels or pels).
At the macroblock layer motion compensation and prediction are performed and it's possible to change the quantisation step.
It must be noticed that, if the picture is an interlaced frame picture, the odd lines of the macroblock belong to the first field and the even lines to the second field.

Video decoding process at the macroblock layer
Decode the macroblock mode and the possible quantiser_scale_code.
If it's an Intra macroblock:
Decode the blocks which the macroblock consists of.
If it's a Non-Intra macroblock:
Decode the prediction mode and the motion vectors.
Produce the suitable prediction for the macroblock.
Decode the blocks which the macroblock consists of, obtaining the prediction errors values.
Add the prediction errors values to the prediction.