What do '(6S)' and '5' mean in (6S)-5-formyltetrahydrofolate?
I've redrawn a truncated version of the title molecule below with the "5-formyl" group in place.  I've also inserted small numbers so you can see how the base system is numbered.  So "5-formyl" means that a formyl group ($\ce{-CHO}$) is attached to the "5" position.  The 6-position has 4 different substituents attached to it so it is a chiral carbon atom.  The R-S nomenclature system is used to describe the 2 mirror image possibilities at chiral centers.  The enantiomer you've drawn is the "6S" stereoisomer.