Much like other American sports, soccer betting has an over/under for each contest played. That would be the total goals scored by both teams combined in a particular match.
Most league games have an over/under line placed somewhere between 2-3 goals, with a popular number of 2.5 for many contests.
A prime example of a soccer over/under line came from a match in the English Premier League this March, when Manchester City made the trip tp London to take on Fulham. Manchester City had been scoring a lot of goals in the league, while Fulham had allowed a record number of goals up to that point of the year. So the over goal line for the contest was set at 3.5 goals at odds of -133. The under 3.5 goals was set at +110. The oddsmakers clearly thought that the match would have at least four goals, since you had to bet $133 to win $100 back (total of $233 returned) on the over total. If you thought the game would be low scoring, you could bet $100 to win back $110 (total of $210 returned).