Have you ever head someone count in a piece of music with "One and a two and a". Why are they doing this?
They're doing what is called counting in. It helps all of the musicians in a group play together it also reveals something special about how musicians create their music.
Counting in beats is a how a musician can measure out sounds, and as such create a specific rhythm.
If we can count beats, we can measure a sound that is one beat, two beats or three beats long, for example.