Mutilities is an app for Google+ Hangout.
It provides a few nice utilities to manage muting yourself and everyone else on the hangout.
When you hold down the "push to mute" button, you will be muted until you release it. You can change this button to "push to talk" by clicking on the gear button. You can also press the "m" key instead of clicking this button.
Sometimes on a hangout its useful to mute everyone else, and that is what the "mute everyone" button does. This is helpful in reducing noise or feedback from an unknown participant while you are talking.