What does He who would gather immortal palms must not be hindered by the name of goodness but must explore if it be goodness mean?

It means don't do what you're told just because other people tell you that's what is 'good', or the 'right' thing to do (and way of doing) it. Think for yourself if it really is right, if it really is good. So it's about being pure of heart and independent, rather than just good at following the rules.