What does ketones in the urine mean?

it means you either have Diabetes , or you are suffering dehydration.

Ketones are formed when the body breaks down fat, so when you are starving and have not eaten enough your body starts breaking down the fat from the cells that is already stored and that is why ketones appear, they are a byproduct of this fat breaking.

Ketones can also appear from uncontrolled diabetes mellitus and rarely after drinking copious and significant amounts of alcohol.
The presence of abnormally high numbers of ketones in the urine (ketonuria) usually results from uncontrolled diabetes mellitus. Ketonuria can also be caused by prolonged diarrhea or vomiting that results in starvation.