Edmonton scored a touchdown to go up by one point with about 30 seconds left in the game. Because it would have been pointless to kick the extra point to go up by two, Edmonton decided to try for a 2-point conversion that would put them up by three points (so a last-second field goal by Calgary would only tie the game).
The Edmonton quarterback threw a pass toward one side of the end zone, and a Calgary defensive back intercepted it and ran it all the way back for a "touchdown." According to CFL rules (and NFL rules too, I think), any pass (or blocked kick that is recovered before it touches the ground) that is returned the length of the field like this counts for the number of points that would have been scored if the offensive team had converted successfully (two points in this case). Calgary won the game by a single point.
Kind of like the Stanford-Cal trombone player ending crossed with the Doug Flutie Boston College Hail Mary.