When doing multi-track music recording, you obviously must mix all the instruments and voices together at the end so that everything is heard and plays well together.
The challenge in mixing is to make sure many different sounds are audible in the right context and in the correct space - both left-to-right as well as front-to-back. One common rule-of-thumb is "never mix with headphones."
More specifically, this advice means you should not mix your music with ONLY headphones. You get a much more accurate picture (odd word to use for audio, right?) of what's going on if you can hear the mix through loudspeakers (monitors), so the sound is coming through the air before it hits your ears.
However, if your space - typically a converted bedroom when we are talking about home recording studios - is less than ideal, mixing with loudspeakers creates its own problems.
Reflections off the walls, ceiling, floor, etc. bounce around and combine with each other, making some frequencies louder than they should be, and others not loud enough.
This creates inaccuracies in what you ultimately hear (constructive and destructive wave interference are the culprits, in case you remember your wave mechanics from school;)).
So there are times when it is advisable to do your mixing with both headphones and with loudspeakers.
Björgvin Benediktsson has some tips for this in his post over at Audio Issues. Check them out here: http://www.audio-issues.com/music-mixing/mixing-with-headphones-use-this-one-trick-for-better-translation/#axzz2ceYlFcI2