When betting on an MLB run line, +1.5 means that the underdog team must either win the game outright or lose the game by one run or less in order for the bet to be successful. With a run line, also called against the spread,  the spread is almost always 1.5 runs.

Betting on the favorite requires your team to win its game by two or more runs. Conversely, betting on the underdog at +1.5 means that your team can either win the game or lose by one run, and your ticket will still cash.