There are some different metres measuring the level of signal: peak meters, rms metres, true peak metres, lufs metres, phase metres.

All audio material has a certain dynamic range between the highest and lowest acceptable levels. We typically arrange for the loudest peaks to be below the maximum level which the system can handle, and for the quietest signals to be kept well above the noise floor, maximizing signal-to-noise ratio. If signals roam beyond these boundaries then your ears will usually tell you something is wrong, regardless of whether you are using analog or digital systems. However, metering can help to make the process of setting optimum signal levels much quicker and easier, warning you of potential problems before they occur.

RMS (Root Mean Square) voltage is a complicated-sounding engineering measure of the average voltage level of electrical signals.  Because the RMS metre measures average levels, a sustained sound reads much higher than a brief percussive one, even when both sounds have the same maximum voltage level.

The reading is dependent on both the amplitude and the duration of peaks in the signal. It is intentionally a slow measurement, averaging out peaks and troughs of short duration to reflect the perceived loudness of the material. RMS metres approximate the way your ear perceives sound levels, your ear will typically not perceive sharp peaks to be as loud as they really are.