In a healthy, non-diabetic adult, testing for ketones= making sure you are burning fat and losing weight.
After you burn your free roaming glucose (created by any carbs you consumed) your body turns to fat for fuel, with ketones being one of the by-products. Since the empirical rule of fuel consumption is glucose>fat>protein, you must burn almost all of your glucose before you can burn fat.
Ex: if you've been exercising and burning fat and you eat a candy bar with 100grams of sugar, your body immediately stops burning fat and switches to the candy bar. (All you biochem majors can double check my math) At 4.5cal/gram and 100 grams, that's 450 calories of carbs you have to burn (4hrs) before you go back to fat burning.
Until recently, you had to use urine pass test strips to see if you had any ketones present. Since you only pee every so often, that isn't a very efficient method (especially if you squat like Fatty). Now you can use a meter and know exactly where you stand... or squat.