This is a simple ground/dirt anchor for vertical trellis vegetable stringing (in my case, tomatoes). Simply tie your string around the knob at the top, then screw the anchor into place near the base of your tomato plant (trying to avoid mangling any roots!). I would suggest using a trucker's hitch knot at the top of the trellis so the tension can easily be adjusted.

Here's a video (skip to 488s) which covers the vertical stringing method for tomatoes. You can either use the tomato clips mentioned in the video (cheap on amazon), or else just wind the plant around the rope as it grows.

I got the original screw thread for this from a series of remixes leading back to https://www.thingiverse.com/thing:3017359 by dbarthel. So head over there for another option to this if you need something with a bigger handle.