Free as in:
- Free Speech.
- Free Beer.
- Free Puppies.
Free Speech: You have the right to do something at no cost, as in "it's a free country" or "free speech."
Free Beer: There are things like "free beer," that might be free for you but someone has to pay for it. (This is something like "everything is free at the library!" but someone has to pay for the materials, staff salaries, lighting, heat, chairs, plumbing...)
Free puppies: Open source software might be something akin to free puppies. You may acquire them at no cost, but you may be looking at some bills down the road for food, vet bills, carpet cleaning, etc.