If the spread is set at +7, the underdog must either win the game outright or lose by fewer than seven points in order to cover. For the favorite to cover, they must win by more than seven points.
The oddsmakers designate each team with the odds to win based on how the two teams match up, the probability of a side winning, and the odds of them being tied after 90 minutes. In the above example, the odds favor the U.S at +145 to win against the underdog Wales at +190.