What an amazing opportunity to express yourself about something as creative and personal as music. You give us the question you're supposed to answer, but I'm not sure what your specific question is for us. My guess is you're just looking for a little direction, so here goes.
What is music and what does it mean to you is a personal, subjective question. That means your answer can't really be wrong; it will simply either be an effective or an ineffective piece of writing. To define music, you may rely on your own feelings and ideas as well as any dictionary or other formal definitions. Brainstorm ideas and jot them down, so they can be reshaped and formed into your definition. I'll remind you that part of defining something may be telling what it is notor what it does not do.
What music means to you is a little more complex, but it's still a discussion of your own thoughts and feelings. A few questions you might want to explore include: When do you find yourself listening to music? Is it when you're experiencing any particular emotions or feelings? Do you turn to music for comfort or distraction or encouragement? Is it something you play or listen to out of habit or necessity? Where do you listen to music most? Perhaps you only listen to it when you're in places where it's being played, such as church or elevators or department stores--if so, why not more? Do you connect most with the lyrics or the music? Is there any aspect of music which is personal and experiential to you (i.e., do you sing or play or write your own music)?
Take one or more of the issues above, add any others you might have in mind, organize your thoughts into some kind of order, create a clear thesis statement, then write. Really, there are so many possible directions for this essay, and they all come down to expressing how you feel about music; and, since the only person who can know that, you're in the perfect position to write this assignment. Good luck!
I think a great way to go about this is taking the time to ask yourself what exactly music means to you. Write it down. Take notes. Describe what it has done for you, some experiences you may have had, experiences you would like to have, what your favorite type of music is and why, and things of that nature.