The Code in hockey is a set of unwritten rules that you essentially have to abide by. Not shoring on the goalie after the whistle, answering the bell after a bad hit and of course, when a play is offside, you stop playing. Every hockey player is familiar with this. If the code is broken, the opposing team doesn't take too kindly to it.
Dallas defender Miro Heiskanen tries to walk the blueline, but accidentally goes offside, instead of the play stopping like it should have, Tkachuk decided to lower his shoulder and deliver a huge hit.
This obviously drew a crowd as the stars players weren't too happy with the late hit, but like I said above, part of Tkachuk's game is to aggravate and get under the Stars skin. This is a play that can't happen though