What is the meaning of propagation??

Dear student,
Please find below the answer:

Propagation means spreading or extension of something into new regions. 
In the case of sound, Propagation denotes travel of sound. The sound travels by disturbing the medium particles. Medium particles vibrate back and forth, to let the sound wave pass through. It can propagate through any medium like water, air, iron etc.

Hope this clears your doubt.
Regards.