Music is made up of sound waves. Those waves behave like those found, for example, in a lake.
If you watch as waves roll into a flat-faced dam on a windy day, you'll eventually see a wave carom backwards off of the dam and directly into another incoming wave. The result is a taller, more massive tower of water. Likewise, the lower parts of the wave get even lower.
This is called a standing wave. If you fashion a container in a specific shape, sound waves will travel in a controlled manner, resulting in a predictable, consistent standing wave that makes a tone. The consistent nature of a tone is what separates it from noise.
So horns and string instruments ultimately help the player make a number of different standing waves. Played skillfully, listeners hear a song.