Clear means clear...
Clear lung sounds means the doctor does not hear the sounds of wheezing
(as in asthma
), stridor (as in croup
or a narrowed airway), rhonchi (as in rattling mucus globs), etc.
.. However, some patients with chronic lung disease (some are preemies, many are not) have lungs that never sound completely clear. One's regular doctor knows this, and might say "clear lung sounds" to mean no new bad sounds.