This is for acoustic guitars that have a pickup but no volume knob/control (like a K&K pure mini or many of the factory-installed pickups in Gibsons for example).
It does NOT require batteries.
Just plug it into the output jack on your guitar and you have a volume knob.
It's called a "JJB Electronics Voljack." Here's a link to the manufacturer's description:
https://www.jjb-electronics.com/Voljack.html