The audio meter shows your microphone level as you speak. Ideally, when speaking normally the volume should peak between the two while lines. If the outer bar reaches the top of the meter and flashes bright red, then it means your volume is so high it is overloading the system and distortion will occur.
To adjust your audio settings, use the AUDIO SET-UP button on the main interface.
If your volume is too low or high, use the volume dropdown to change it. The mic can also be switched off here.
To mute computer sounds (e.g., YouTube videos you are capturing) switch the MUTE button on.