Bonus is the currency used in BBGsite Contest. And also, If you have BBG/MMO Gold, you can exchange Gold to Bonus. 1 BBG Gold = 500 Bonus.
See the red frame shown in the pic below? A higher level will lead to more Bonuses as well. Initially, the amount of EXP equals that of Bonuses. Surely, you can also obtain or consume Bonuses in other ways!