"post" means after
"bac" means bachelors
"postbac" means anything you do after undergrad
For the purpose of med school admissions, postbac implies that you decided to go for med school after you already had a bachelors. The context in which you get the prereqs done is a detail buried in a detail. But the school where you get the prereqs done does matter. For example if you are doing a health education masters at a chiropractic college that offers the prereqs, that would not be the same as doing UNM or similar.
Be aware that you may be required to categorize the prereqs as grad work, which means they would not be reflected in your undergrad GPA. This may or may not matter, depending on your undergrad numbers and MCAT. Generally a good grad GPA is neutral, nowhere close to the weight of undergrad GPA. See the very comprehensive AMCAS instructions for more info - don't take anybody's word for it when there's such a good official resource.
Best of luck to you.