Humans are the result of millions of years of nature's engineering, so it's not surprise that our bodies can do some pretty amazing things. One of these things it what psychologists call the "cocktail party effect." This is the ability to distinguish and pay attention to one particular voice out of a multitude of sounds.
Our ears take in a huge number of different sounds, often at the same time. Sometimes it's overwhelming, but if you close your eyes and focus, you can almost always pick up on and follow one particular sound. It's a feat of nature that let's us make sense of the world by creating order from chaos.
When concentrating on one sound, our minds are essentially declining to process the other sounds. If you're at a cocktail party, for example, and tune out of your conversation to listen to the conversation next to you, you're going to miss what your group is saying. In this way, the ability to pick up on one particular voice and the ability to tune things out are very closely related.
