As explained in Audio Mixing in Timeline Preview [VE UG] the VidiEditor timeline preview mixes all audio channels of the timeline into a mixed audio signal output when playing the timeline. Whatever channels are played back will be also previewed in a peak meter view that can be find on right hand side of the timeline.
This indicator helps the user to judge on the output signal of the timeline playback and also gives prediction for the output signal generated on timeline publish. It displays the level of all channels the user also hears when playing the timeline.
The peak meter displays the mixed stereo signal in 2 bars and in dBFS scale.