A +1.5 in a baseball betting line means you can bet on that team with 1.5 runs added to its final score.
For example, if you bet on the New York Yankees +1.5 against the Houston Astros, and the Astros win the game 3-2, your bet on the Yankees win. If the Astros win the game by two runs or more, you lose the bet.
You can also win the bet if the Yankees win the game outright.