I seem to recall that the -- is a way to tell Git to treat what follows checkout as a file and not as a branch. Suppose that you had both a file and a branch called stuff. Then the following command would seem ambiguous:
git checkout stuff
because it is not clear whether you are asking to checkout a file or a branch. By using -- you explicitly tell Git to checkout a file by that name/path. So in this case the following commands allow checking out a branch and a file called stuff:
git checkout stuff # checkout the branch stuff
git checkout -- stuff # checkout the file stuff
Note that git checkout <name> is really meant for branches, but Git syntax is relaxed, and if Git can’t find a branch, then it will look for a file.
Closely related: Git change branch when file of same name is present