D Mannose is a form of sugar, although I believe not metabolised by the body in the same way as other sugars. For those who are prone to utis it can be a very good thing to use every day, prophylactically, as well as more intensively when in the throes of an acute infection. It doesn't work for everyone, unfortunately, and I think is only effective against e coli infections (although e coli is responsible for the vast majority of utis).
It broke a long cycle of recurrent utis for one of my daughters so, when it does work it can be a lifesaver.