I'm currently using this script but have a question about the last parameter, EV Correction in 1/3.

What exactly is this suppose to do or what is it used for?

Perhaps I missed it, but I didn't see a direct answer to this question.

EV Correction basically decides the "starting" or "center" exposure in your set, expressed in 1/3 EVs. (So a full -12 or +12 would represent -4 or +4 exposure.)

For instance, if you had EV Correction at -6, then that would start your exposures at -2, stepping up or down from there.

Let's say you set your step to 3 (or 1 EV) and set it to take three photos.

If your EV correction is 0, then your three photos will be -1, 0 and 1.

If your EV correction is at -3 (or -1 EV) then your three photos will be -2, -1, and 0.