Here the condition is expressed by

Here the condition is expressed by the clause if it rains, this being the protasis, while the consequence is expressed by the picnic will be cancelled, this being the apodosis. (The protasis may either precede or follow the apodosis; it is equally possible to say "The picnic will be cancelled if it rains".) In terms of logic, the protasis corresponds to the antecedent, and the apodosis to the consequent.