The input button on a Vizio Soundbar is used to switch between different input sources. Depending on the model, your Vizio Soundbar may have different input sources, such as HDMI, Bluetooth, Optical, or AUX. By pressing the input button on the remote, you can cycle through these different input sources and select the one you want to use to play audio. Some Vizio Soundbars also have an auto-detect function, which automatically switches to the input source that is currently active.