This type of bet is based on the total goals of a football game. The bookmakers set 2.5 goal as the line and the punters choose under 2.5 (no-score, 1 goal and 2 goals) or over 2.5(3 goals and more).
There is no draw possibility of this bet. It means you will win or lose your stake at the end of 90 minute time. You have 50% chance to win the bet. No extra-time or penalty shoot outs after 90 minute counts for this type.
Manchester United vs Arsenal - Under 2.5 = 1.95 - Over 2.5 = 1.90
Consider that you bet 100 on Under 2.5 goals. If the game ends with 0,1 or 2 goals at the end of 90 minutes, your stake will return as 195.
It means you have 195-100 = 95 profit. If the game ends with 3 or more goals, you lose your stake.