The "mAh" (Milliamp Per Hour) number usually found on the face of the batteries is referring to the capacity of the battery. In simple terms, this is the "fuel tank capacity" - the bigger this number, the more run time you will have from your RC. However, it's important to note that as the number increases, so does the battery size and weight. This is important to remember when it comes to battery tray dimensions (will the battery actually fit in the RC) and it will also influence flying weight and center of gravity in aircraft, and weight bias in boats and cars.