0dBu is a voltage reference point equal to 0.775 Vrms. It used to be called "dBv" but that was confused with "dBV", so it was changed to "dBu", where the "u" stands either for "unreferenced" or "unterminated", depending on the document.Pro systems are generally referenced to +4 dBu, a level equal to 1.23 Vrms. The "rms" or root mean square reading is important, too. You have to use a meter that can do this to get accurate readings.