This is the mathematical advantage the game gives to the online casino, ensuring that the online casino makes a definite profit from the game in the course of time.
To calculate the bonus EV, you simply have to deduct the bonus amount from the product of the house edge and the wagering requirement.
For example, if you received a bonus of $100, which has a wagering requirement of 40x and you use it on a game with a 3% house edge, your bonus EV is -$20. You should always run away from bonuses with a negative EV as they will make you lose your hard-earned money in the long run.
Let us now consider another example. You have received a bonus of $500, which has a playthrough requirement of 20x and you use it on a game with a house edge of 3%. In this case, you get a positive EV of $200. This is a great bonus to claim as it will actually help you to boost your casino bankroll.
And if you are bad at math, you need not to worry as you will find plenty of high-quality bonus EV calculators online. You can use these absolutely free of charge provided you know the wagering requirements and the house edge of the game you intend to use your bonus on.