The play occurred after Boston Bruins forward Erik Haula had won a draw that MacKinnon thought may have been unfair, and when the official did not blow the play down to re-do the draw or waive Haula out, MacKinnon delivered the chop. You can see the play below.
Per NHL rules, abuse of an official carries an automatic 20-game suspension, but it is worth noting that there are subsections of the rule that would allow the NHL to suspend MacKinnon for 10 or even 3 games. Needless to say, this one was a pretty clear, purposeful infraction. I'd be surprised to see him get any less than 10 games.
While it's really anyone's guess as to how the NHL will view this, I have a hard time believing that Mack will get less than the 6 games that the league just issued to Brad Marchand for punching and high-sticking Tristan Jarry on February 8th. At least with Marchand, he was targeting a player and not an official. Both dumb decisions, but one was definitely worse than the other.