PRC Smoke Stopper (XT60)
This simple device is designed to connect inline between your battery and your drone, and will protect your electronics in case of any electrical short circuits. Whether it's a bad solder joint or a mismatched wire, the Smoke Stopper will cut the power and notify you of any shorts.
Green means go - if the LED lights up then your circuit is good. If there's no light, that means that there is a short in your wiring. Once the internal fuse has been tripped, it will take a minute or so to reset; during that time the LED will come on dimly when powered.
Note: The Smoke Stopper is just one of many tools that should be in your electrical kit, and is not a magic bullet. It will not protect against all wiring errors, and you should always check your work with a multimeter before applying voltage.