In science, to denature something is to transform its basic qualities in a significant way. When you cook egg whites, you denature them, changing them from transparent and liquid to solid and white.
Chemists frequently denature proteins, and cooking an egg white is one example of what that can look like. Another is the skin that forms on top of milk if you leave it out of the fridge for a while. Substances can be denatured by heat, acidity, and various chemicals. Adding nasty-tasting or poisonous ingredients to alcohol so it can't be consumed is another way to denature something. Figuratively, this word means "alter": "Taking away recess denatures the school experience!"