Yes free clinics are really free and they're available to people with limited income sources, no health insurance and to those people who do not qualify for Medicare or Medicaid.
You do have to sign some paperwork though to get the free clinic services so you cannot really just walk in but you have to show paperwork to them to get qualified for the free clinic beforehand and setup appointments with them.
But after you qualify for the free clinic services then yes it really is free and you won't be billed for the services however free clinics are not as good as regular clinics that are paid for by health insurance or out of pocket but they're not bad clinics either and are better than no clinic at all.