This is a holder for pre-soldered WS2812b "pixels". The holder accepts one "pixel", which is a PCB with a WS2812b soldered on it. These are typically found as a pre-soldered string, like this.
This holder by itself isn't very useful - you probably don't want to print this directly. Yo\u most likely want to integrate this into your design. If you want to test this, you can print the "LED holder disk.stl", which is the holder on a small disk. You'll also need the top piece, which snaps onto the bottom. If printing in PLA, use the "LED holder top.stl". If printing in PETG, which is a bit more flexible, I recommend using the "LED holder top tight.stl", which has slightly longer protrusions. You'll probably want to use use a translucent filament, y'know, so you can see the LED. I like translucent/natural PETG.
I've only tested this with a 0.2mm layer height.