The House Multiplier option is located in the "Houses" section. By changing this value higher than 1 it creates a multiplier, meaning every Coin contributes it's point value times this multiplier to House scores when earned.
This feature is great for temporarily incentivizing students, for example the day before the end of the Round or the day before a big test.
TIP: The House Multiplier only affect House points awarded, it does not increase the reward balance earned for each coin.