静音 is a noun that refers to the 'mute' function or 'silent mode' on electronic devices (phones, TVs, etc.). Despite its literal meaning of 'quiet sound', in modern usage it almost always means silencing a device. You'll see it on buttons and in settings menus. Colloquially, it can also be used as a verb (e.g., 静音手机 'mute the phone'), but the dictionary POS is noun.