Farm Heroes Saga’s Hero Mode is similar to Sugar Rush in Candy Crush Saga. If you complete the goal in Farm Heroes Saga before your moves run out, you will enter Hero Mode. During Hero Mode, every move you make will give random cropsies a number of bonus points. You can make extra matches with these bonuses to get an even higher score and reach three stars for that level.
Checkout this Farm Heroes Saga video on how to effectively use Hero Mode to the best of your ability!