This is an expo marker holder that attaches to a specific model of rolling whiteboard. It supports holding four markers and has a platform on the top where an eraser is intended to sit.
I have come across two nearly identical models of this whiteboard with differing widths (14mm & 15mm). The stl file that I have uploaded is for the 14mm model. If the marker holder doesn't hug the whiteboard snugly I recommend putting a piece of masking tape on the board and sliding the marker holder over that.
Here's a link to this design on tinkercad.