What does it mean when the cars air conditioner only puts out warm air?

Obviously, it means the AC is not working. You can hope that it only needs to be re-charged with coolant. However, coolant should not leak out, so unless the leak is slow, just adding coolant will not help for very long. Leaks can be found and sometimes plugged. Sometimes the electrical controls fail. The worst and unfortunately not uncommon problem is that the compressor must be replaced.