There is no simple definition of music. But in general terms, it is an art form whose medium is sound and silence.
Sound and music
Sound is a vibration. When an object vibrates, our ears pick up those vibrations and send them to our brains. Then, our brains analyze those signals and we perceive them as sounds.
Sound is an important aspect of music. But, not every sound is musical. In fact, some of them are noises. Musical sounds have certain elements. We will discuss them later. Noises have no or lesser musical qualities. If we want to compose a piece, we should organize musical sounds by rhythm, melody and harmony. So that, they can be pleasant to listen to. Just like sound, silence is important too. We need some breaks between notes. This is why we use rests. This constant use of sound and silence what makes a composition exciting.
Music is a language that musicians use all the time. If we want to use compositions for future performances, we should write them down in a way called musical notation. Musical notation consists of instructions about how and when to perform a piece. This is how musicians communicate with each other. Every musician can understand and use that language to recreate music with each performance.