It's just a very basic tool to generate custom nuts and bolts. Useful for 3D printing since the only format you can export is STL. I wrote this while OpenSCAD was busy rendering a simple screw for me <3
You will need to have a Java Runtime Environment (one that ships with javaFX) installed on your computer to run Thread2STL. If thats the case then download the Thread2STL.jar
from the releases page. Since it's an executable .jar file either double click will be sufficient or
java -jar Thread2STL.jar
should do the trick.
The STL files are in ascii format which is why they become quite large depending on your preferences. Also not every other program can read this format.