This is a pack of two Maclan Max Current 5mm to 4mm Bullet Reducers. These reducers are great if you typically use 4mm bullets, but happen to have a LiPo with 5mm bullet holes, and don't want to put 5mm bullets on your ESC. Instead, install these reducers into the 5mm hole in your battery, and now they're compatible with a 4mm bullet connector. 

Also available in an eight pack: MCL4169