When the carbohydrate intake is insufficient to meet the body's glucose needs, it switches to a different metabolic pathway that breaks down fat and protein. The products of this alternate pathway are ketones, most of which can be used in place of glucose. However, there is one ketone, called acetone, which we cannot use, and instead, excrete it via the kidneys. The presence of ketones in the urine is called ketosis. This isn't necessarily a bad thing -- for us, it signals that fat is being broken down by the liver (yay!). People who are insulin-dependent diabetics, however, have to be careful, because they can develop ketoacidosis, which is the dangerous version
Drinking enough fluids should keep the acetone levels in the urine at reasonable levels.
The link below describes it in layman terms.
What is Ketosis? Is Ketosis Dangerous? How to Detect Ketosis