Infant skulls has dense, fibrous connective tissue joint called sutures that connects different sections of skull (temporal, parietal, occipital). It allows little movement and makes the skull slightly elastic in order for easier birth and will harden after a while.
Craniosynotosis is when those sutures prematurely harden/ossify (turn into bone). It disfigures the formation of the skull and forces the facial structure and brain to grow out of shape due to the lack of space. It may contribute to visual impairment, deafness and low IQ.