What is a focal disc protrusion with cranial migration mean on a spinal mri?

It is: a disc herniation, which means disc material has been displaced beyond the normal confines of the space between the vertebral bodies. Technically, it is a "contained" herniation that is still covered by the anulus fibrosus, whereas a disc extrusion extends through the anulus. Cranial migration means the disc material extends upwards(in the direction of the head) from the lovel of origin.