It's basically about eating foods together that digest well together. For instance, carbs need an alkaline environment to digest properly, but meats need an acid environment. If you eat these together the environment will become neutral and neither will be digested properly, leaving them to rot and ferment. However, non-carb veggies can digest in either acid or alkaline environments so you could eat them with either carbs or meat.
Once you learn which foods combine well, it's pretty easy to follow.