What does it mean when people say "fair share?"

Fair share:  It's a catch phrase liberals use when someone is doing something they don't think is right.  For example, if you make too much money and don't pay enough taxes, you can't pay your "fair share."  It does not refer to the poor not paying taxes.  If you said, "The poor aren't paying taxes, that's not fair.  The poor aren't paying their fair share," that is not an appropriate use of this catch phrase.  If you said, "The rich are paying too much taxes," that is not an appropriate use of the phrase either.  It only refers to things they don't think are right.