This is the statement made by ChoiceOdds with which you agree or disagree and you bet accordingly. It is essentially a description of the bet being offered.
For example, the proposition may be "FTSE to up more than 50 points at 16.30". For a binary bet you would "buy" if you agreed with this proposition or "sell" if you disagreed with it.
ChoiceOdds is licensed and regulated by the UK Gambling Commission. License no. 000-002667-R-103780-001