When laying a bet there are various options….
You can underlay so that if your bet wins you are better off than if it loses. This can be popular to give you an interest in the event whilst minimising your risk.
Alternatively you could overlay if you think your bet is likely to lose and you want to profit more if you are correct in this.
The bet calculator can also be used in advanced mode to see what the results would be for different levels of under/overlay. Simply enter the odds as usual and then use the bet slider at the bottom to illustrate this.