A Run Line bet is the point difference set by PointsBet between the two teams. The favorite needs to win by more than the line to cover, while the underdog can lose by less than the line (or win outright) and still cover.
For example: the Blue Jays are a 1.5-run favorite against the Tigers. The Blue Jays must win by 2 or more runs to cover the spread. If they win by 1 run, or lose, the Tigers cover.