It is a phosphoric acid based solution and works well. There are other brands that work as well. They turn the rust into a black compound that can be painted over. Remember it is acid so don't get it on your skin or clothes.
It's also commonly used to rub down new metal, such as floor pans after the welding has been done. Wipe it on with a red scotchbrite and then wipe away residue with some thinner. Fresh metal will stay clean and well preserved until you can get some coating on it .