Db stands for decibel. It is a measure of noise. For instance, the typical airplane takeoff is around 130db to 150db. An average conversation is 60db. So the noise reduced is a blocking of x number of decibels. This can help prevent hearing loss by allowing you to play music at lower volume levels for your own hearing comfort. Conversely, some people complain when the db reduction is too great because the silence can cause vertigo problems or make inner ear issues apparent to the listener.