The translation of osteoderm is bone (osteo) in skin (derm), which is exactly what they are. Osteoderms are the result of bone tissue forming in the skin.
This occurs in the scales of crocodiles, alligators and other reptiles, and in mammals such as armadillos. Osteoderms provide armour- like protection against predators. A rolled-up armadillo, for example, is almost entirely encased in bone.
And it's not just predators they offer protection against as osteoderms have also been seen to deflect bullets.
In crocodilians, osteoderms also aid temperature regulation, containing a network of blood vessels that can transfer heat to and from the animal’s surroundings.
Dinosaurs also had osteoderms