It's quite simply a stick (or tip) to open and close window air vents common in the EU.

There are two models: 

• A small stick that works fine if you can easily reach the window and is also printable on a Prusa Mini+ (diagonally on the bed)
• Just the piece that connects to the window with a small (1.2mm diameter, 10mm deep) hole so you can attach it to a longer stick using some threaded rod.

I recommend printing it on its side as the photo of the print bed shows. This ensures the nibby bit is strong enough. If you would print it with the nib sticking up, it's quite easy to break when closing or opening the vent.