Apple has a very complicated defense system. When you cut the apple, its surface cells are damaged. An enzyme tyrosinase (also k/a “polyphenol oxidase”) is released from the cut cells. This enzyme is an oxidase. In presence of oxygen (from the environment), the “o-diphenols” of apple convert into “o-quinones” or ortho-quinones. These o-quinones further react with oxygen to form melanin; which is brown in colour; hence the brown color.
Remember, this is the same melanin which gives coloration to human skin.
These o-quinones have antiseptic properties; they provide protection to the apple from bacteria. So, if the apple is brown, means it is microbe free.
Yes, there is an easy way to get rid of this browning. Place the cut apples in salt water for about 5 minutes. The acidity of salt water will denature the enzyme, tyrosinase; and stop the overall reaction. But remember; now these cut apples have been exposed to microbial action.