This is a 2 Cell (in series) 18650 Lithium Ion Cell Holder with a lid and switch.
Caution! It's just a "dumb holder", wire, plastic, tab and spring, there is no protection or charge circuitry at all.
If you are using Lithium based cells, make sure you know what you are doing and take all appropriate safety precautions!
18650 size cells (and close relations) are often found in laptop battery packs, and Tesla Roadsters amongst other devices.
When a laptop pack dies, often (usually) just a couple of the cells are dead and you can very carefully take it apart and remove the good cells from it to put to work in other ways.
So if you have an old laptop battery pack, or a no longer required Tesla Roadster parked in the garage - you might have some use for a cell holder like this to make use of the old cells.
