This is the 108th most frequent Polish word.
mu
The Polish word "mu" means "to him" or "to it" (dative case of "on"/"ono").
The word "mu" functions as a dative case personal pronoun meaning "to him". It indicates to whom the book is being given.
Dałem mu książkę.
I gave him a book.
The word "Muzyka" in this sentence means "music". Note that "mu" is not used as a word here, but it appears within another word to illustrate contexts.
Muzyka gra w tle.
The music plays in the background.
The word "mu" in this context is used as a simplified replacement for "temu zdjęciu" (to that picture), showcasing colloquial brevity in Polish.