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 odds are the ratio between the amount staked by the bookies and the bettor, so 7/1 means the bookies stake seven times the amount the bettor has wagered. If the bettor wins; their predicted outcome materialises; they will take seven times their bet from the bookie (in this case).