Complex sentence: Countershading, or Thayer's Law, is a method of camouflage in which an animal's coloration is darker on the upper side and lighter on the underside of the body.
Split into simpler sentences: Countershading, or Thayer's Law, is a form of camouflage. Countershading is the pattern of animal coloration in which an animal's pigmentation is darker on the upper side and lighter on the underside of the body.