It’s just a temporary stiffening of the muscles in the body after death. It basically makes a body go rigid for ~1 day after death, and then it goes away again.
One cool palaeontology related fact: If you look at how many small theropods (meat-eating dinosaurs like coelphysis) have fossilized, their pose always seems to be similar. The neck and tail are curved into a tight arch. Palaeontologists think this is because the tendons and ligaments around the spine dry out and contract after death, and pull their backbone into this shape. The same can be seen in modern birds left outside to dry out.