Please Read - FORUM RULES!
As a song plays, the output meter on the stereo/main out bounces up and down with the music. You can see how high (loud) an audio track peaks by looking at this meter. Anything peaking over 0 level usually means distortion, so at all costs stay below 0 level.
How far is your peak loudness below 0 level? That's the simple definition of what headroom is. So, if your meter is peaking -3dbs below 0 level, you have +3dbs of headroom. If your meter is peaking right at 0 level, you have NO headroom.
Note - +3dbs to +6dbs of headroom is the standard recommended amount, but many times a song with more or a little less headroom can still be mastered with no problems. But, it really makes no sense to give a mastering engineer a mix with 0dbs of headroom and risk the chance that it's slightly distorted. There's no reason to do it because overall song volume is done in the mastering process, not in mixing.