It refers to the resin in a marijuana smoking device that's built up for far too long, accumulating over the years traces of various other drugs including but not limited to: Cocain, LSD, Opium, etc. This stuff is evil in it's most concentrated form, but Satan sure is one hell of a crafty temptress.
Frank: me and Sam scraped my dad's bong last week
Rob: I'd be scared to smoke that devil tar
Frank: lol, he ddin't like it