The implication of "storefront" is low academic standing, poor academic repute, a "start-up" with no academic credentials or distinction. It would not have a campus, or a quad, a library, or dorms, or any of its own buildings really -- just some rented rooms on the street level in the business or shopping area of a city.
A storefront law school would be not accredited, or barely accredited. Law colleges, chiropractic "colleges," and schools of divinity are the most common ones in this class. Medical colleges of this type were drummed out of the US by the AMA, I believe.
"Storefront colleges" are on a par with "correspondence course medical degrees" -- at least in the vernacular use of the term.
Here's a website that says it has statistics to show that some storefront law schools and other unaccredited schools cost less yet perform better than some of the schools with ABA accreditation.
Student or Learner