I'm not sure the exact formula used it for, but the number is a general representation of how difficult it will be for the team to make the playoffs. The closer it is to 0 the more unlikely it becomes. I imagine it's a mix of home / away, scheduled games remaining and comparative to other teams in the hunt.
The tragic number is the inverse of a magic number. It's the number of points the team can squander and the number of points the last playoff team needs to earn in order to eliminate the team.
Basically, the Blue Jackets must win every single remaining game and Boston must lose every single remaining game (in regulation) in order for them to make it, with a tragic number of 1. They can only get to 90 points, and the last playoff team (Boston) is sitting on 90. A single lost point in either direction eliminates them.
On the flip side, Tampa Bay needs one single point (or for Ottawa to miss out on one more possible point) to clinch because Ottawa can currently finish no better than 100 points and cannot catch up to Tampa in ROW.
Meanwhile, the Rangers have sent Chris Summers back to Hartford, so I guess Hunwick is fine.