Meters measure voltage or level of signals. 
Meters offer a means for the audio artist to tell how much signal is passing through the various stages of the signal path. This is important because any piece of audio equipment has limits in the level of signal it can pass without either undue noise (low signal levels) or distortion (exceedingly high levels), so a means of determining is signal levels are optimum is welcome. 
Three kinds of meters are commonly used: 
peak meter     vu meter     peak indicator 
Peak Meter 
The peak meter responds almost instantaneously to changes in the signal level and is effective in displaying potentially problematic peaks or transients which may exceed distortion limits in the device being used. Because the instantaneous fluctuations in a signal may be so brief as to pass unnoticed, most peak meters are designed to react as fast as possible to peak onsets but fall more gradually from these peak levels.   

The human ear however is not particularly sensitive to instantaneous peaks in signal level and while such peaks of short duration may be present in a signal the perceived loudness of the overall signal may not sound that loud to the ear. This is because the human perception of loudness is based on a kind of averaging of sound wave levels with dynamic changes persisting less than 200 ms being progressively less perceivable by the ear as their duration decreases. This makes a peak meter less effective in indicating how loud the signal being passed might sound to the human ear. 

VU meter 
A more effective meter for communicating loudness is the VU meter. Calibrated in volume units the VU meter reads somewhere between the average and peak values of a complex waveform. 


It must be kept in mind that while the VU meter gives a closer approximation to our subjective sensation of loudness, the actual signal levels being measured will be consistently higher and that some instantaneous peaks may be high enough to cause distortion or peak clipping in the devices being used. 
Peak indicator 
Many pieces of audio equipment provide a small LED (light emitting diode) indicator known as a peak indicator. This LED will light if signal levels approach or exceed distortion or clipping levels.