Over/Under 2.5 goals is a common betting line for the total goals in a soccer match. Online sportsbooks set this line in anticipation of how many goals they think will be scored throughout the duration of a game.
For example, if Manchester United is playing Manchester City the number of total goals could be 2.5 goals. Now, of course, there is not going to be a half-a-goal scored in the match. But that .5 means that there will be winners and losers betting on these odds and no “pushes’’.

If the final score of the match is two goals, one goal, or a 0-0 draw, then the under 2.5 bet wins. If the total score of the match totals three or more goals then the over bet wins.
For example, if the final score of the match is Manchester City 1, Manchester United 1, then the under is a winner, and the over is a loser.
If the final score is 2-1, with either side winning, then the over wins.
This is one of the more common wagers you will find in soccer betting.