What you think it means: These chickens were living in a barn or outside and had free range to engage in their instinctual foraging behavior.
What it actually means: It just means that chickens are not living in cages. “Cage-free” chickens can still live in aviaries (massive industrial barns with thousands of birds), with each chicken having, about 1-1 ½ square feet of space. Granted, they are still better off than caged counter parts, but still not great.
Take Away: At the end of the day, unless you are raising and slaughtering meat yourself, there is really no way to have a GUARANTEE that your meat was treated well, fed well, and had a happy life. I have found the closest thing to a guarantee is to find a butcher you trust, look that person in the eye when you ask the tough questions, and just they are not pathological liars.