As with many other chronic illnesses, chronic kidney disease is measured in stages. These stages mark the progression at which the kidneys are losing function.
Depending on your stage of renal failure, these may be simple changes or very strict restrictions. This is because what you eat has a huge effect on your kidneys.
Everything you eat, after all, ultimately gets filtered through your kidneys. As your kidneys lose function, it gets harder and harder for them to filter out waste and other excess material.