 Many Adafruit boards, like these feathers, let you easily connect a rechargeable battery for portable projects. It's a helpful feature. Just look for the two-pin socket, usually labeled lipo or lipoly. Of course, Adafruit also sells a lipo battery with matching connector. It would be great if everyone followed the same standard. Unfortunately, there are vendors online that sell batteries that look very similar, but have the red and black wires swapped. For example, this one is correct with the red wire on the left when plugging it in. But this battery has power on the right and ground on the left. Reversing power and ground like that can damage the feather, so keep an eye out and mind your power connectors. Thank you.