"Crackles" is just a term like many others to describe adventitious
sounds heard in the lungs. Like many terms in medicine, it may mean
different things to different people.

Crackles are typically regarded as different than wheezing. Crackles
can best be mimicked by rubbing your a small clump of your hair
between two fingers right next to your ear (if your hair is long
enough). While not entirely specific, "fine" crackles tend to indicate
fluid in the alveoli whereas "coarse" crackles may result from mucus
or fluid in the airways. Crackles are typically heard on inspiration.

Wheezing, by contrast, is typically thought of as polyphonic, musical
sounds. That is, one hears many sounds simultaneously that have an
actual "note", as opposed to the staccato white noise that makes
crackles. Wheezing is the usual sound heard from bronchospasm. Also,
unlike crackles, wheezing is typically heard on expiration.

Crackles may be from pneumonia, but other lung conditions can cause
crackles as well. An X-ray can help differentiate these to determine a
proper diagnosis and thus proper treatment. The answer to "why not an
antibiotic?" is because it might not be a bacterial infection.

PF