This is not intended to be a flame, but any time you have a question, simply enter it into your browser search. For example, the search for "What is reference level" produced many hits, including this appropriate one from the Audyssey site:
ps - if listening at "0db" on the AVR is reference I'll have a hard time ever watching it at that level since with my new AVR and speakers I have a hard time getting higher than -5db since it is very very loud (and I'm one who likes it loud)!
Most people find reference level too loud in a domestic environment. I listen loud too, often at -5dB or so, and I know I couldn’t really take any more for very long - and this is in a system/room that has substantial headroom so it is clean all the way up to 0dB. It is interesting that AVS-er 'FilmMixer' - a professional film mixer doesn't listen at reference level at home as he finds it "too loud".