The vox-l simulator is the testing area that you use to test your mods within the browser.
The vox-l artifact is the most uptodate, most likely to be bug-free, version of the vox-l simulator.
(You can get to the artifacts by clicking the red square on your learntomod homepage).
Sometimes the version of the vox-l simulator that runs by default when you use learntomod for the first time, isn't always the most update version of it. Unlocking the vox-l artifact solves that problem.