'If need be' allows you to indicate that you can make a selected date if necessary (i.e. it is not your preferred date and time to meet, but you could make it if you have to).
Click on an options to vote yes.
Click on the same option again to vote 'if need be'. You will see that the option turned yellow to indicate your 'if-need-be' vote.
Finish by clicking 'Continue'.
