This is a connector clip (printed in RED in the photos) for the Bambu Lab Spool Holder model created by @Biskup. The clips connect together the spool holders so they don't all slide around when you use them.

You will need to print Biskup's model in order for these clips to be any use. I recommend using 2 clips to connect each of the spool holders, but 1 should be enough if you don't want to spend the time and material printing. I used PLA in the test prints, but I'm sure ABS and PETG would be fine too, but aren't needed. The clips grip pretty well.