'Chilling' has various meanings depending on the context and usage. In formal English it represents 'Coldness, Becoming Cool, Falling in Temperature' or 'Scary, Frightening'. But in the form of a 'Slang' it is generally used to mean 'Relaxing, Becoming Calm'. So in your sentence the context is 'listening to music'. Its a common usage in informal & casual English (Slang). It would mean 'Relaxing, Becoming Calm, Soothing the Senses'. 'Doing nothing very serious but just relaxing while listening to music.' Hope it helps.

relaxing by listening music??? i guess

"To chill" is a common slang meaning to relax, ie. become calm. So you are relaxing while listening to (relaxing) music. Another slang form is "chillax" (chill+relax).