"Sacralisation", for those who don't know, means that the vertebrae are partially fused. The sacral and coccygeal parts of the spine - the lowest parts are actually vertebrae which are separate in children, and fuse as we grow up (otherwise we'd fall over when we tried to stand up).
C = cervical (neck), Th = thoracic, chest and upper/middle back, L = lumbar, lower back opposite abdomen.
Discs sit between the cervical, thoracic and lumbar vertebrae, basically as padding between them; if you have a partial fusion, scoliosis, lordosis or similar, then the vertebrae are a bit wonky and squash the discs on one side, maybe even making one pop out (herniation, painful).
In about 10% of the population, one or more of the lumbar vertebrae are partially or fully fused, too. Normally this requires no treatment. However, the partial fusion can push the disc out a bit between the sacral and lumbar vertebrae, or can make a larger gap between the bottom two lumbar vertebrae so that the disc jumps around a bit; because the bottom one is held down by its attachment to the sacrum, it makes a gap.
This can make some of the muscles of the lower back, hips and abs tight, and some loose, as the muscles try to pull everything together, but get stretched, and so on.
This is apparently what lazy has got. I dunno what his specialist will advise. Usually it's some light lower back strengthening exercises, things like the "bird dog" - get on all fours, point one arm forward and the opposite leg back, hold for ten seconds, change over, etc.
Quite treatable without anything dramatic or painful in most cases, as I understand it, but his specialist will have lots more to say. Thanks for sharing the cool pics with us, lazy