Its a wire spool holder that works with Gridfinity, its uses 2x4 grids.
It has a bit of storage for screws, nut, bolts, small wires, pens etc.

The spools should be about 18-19mm wide and a diameter of about 60mm, the inner diameter should be 9mm or more, so it fits the shaft.

I have used these spools made by Jay, they work amazing with this, but you can use any spool that are within the requirements.
 

List of the requirements:

OD: about 60mm
ID: 9mm, larger will also work, but smaller may not.
Width: Ive used 18-19mm so i can fit 8 spools, if you use wider spools you may not be able to fit as many.
Gridfinity baseplate that has a 2x4 grid or larger.

Link for the spools made by Jay.
[https://www.printables.com/model/483237-wire-spool](https://www.printables.com/model/483237-wire-spool)