Cuzz "0" is supposed to be your ideal level...and + is where you start getting into tape saturation and distortion...Below "-" (minus) is leaving your headroom open and is taken up by tape noise (assuming your using analog)
Depending on your recording platform +3 to +6 is ideal.....I'm sure there are lots of technical proffessionals on here who could do a lot better job explaining it all than me...(and there are lots of REAL recording engineers on here) But that's the basic jist of it...hope this helps