The cultural knowledge that you have about music is your knowledge about how the music fits into culture and society. Who would play a piece of music? When and where would they play it, and how would they be dressed? What are the reasons and the rules for playing this kind of music? Who would listen to it, and how would you expect them to react?
If you are not a musician yourself, you may find the cultural aspects of musical performances to be among the most interesting, as well as the easiest for you to learn about. Even if you are most interested in the music for its own sake, separate from the people who create it, you will find that cultural aspects of the music have important effects on the music itself. For example, songs for dancing usually have a strong and regular beat, and popular songs tend to have a slightly-repetitive refrain that is easy to recognize and remember.
Even if you believe you have no knowledge about music at all, you almost certainly have some cultural knowledge about the music traditions around you. For example, if you know what month "Silent Night" should be played, why the audience claps in the middle of live jazz performances, how to dress for a Beethoven concert, or where to go if you want to dance on Saturday night, that is cultural knowledge about music.