Over / Under betting is based on whether the total number of goals scored in a match is:
over a specified line
under a specified line
The most common line in football is 2.5 goals.

Why 2.5 goals is common
Football matches cannot end with half goals.
Using a .5 goal line ensures there are only two possible outcomes:
Over Under
This avoids draws and simplifies settlement.

Example of an Over / Under market
A typical Over / Under 2.5 goals market might look like this:
Over 2.5 goals → 1.95
Under 2.5 goals → 1.85
If the match ends:
3 goals or more → Over wins
2 goals or fewer → Under wins
Other common goal lines
Bookmakers may offer different goal lines, such as:
Over / Under 0.5
Over / Under 1.5
Over / Under 3.5
In general:
lower lines → higher probability, lower odds
higher lines → lower probability, higher odds