It means a couple of things
Generally, it means that you sign up to play at a certain casino intending only to play long enough to keep some or all of a bonus they offer when you sign up. In other words, you never really intended to keep on playing there for a long time. You're just there because of the bonus.
Also, it implies that you're not really playing on-line BJ intending to beat the game. Your goal (as I said before) is to retain some or all of the bonus. That may mean that you actually lose a portion of your original investment, but (hopefully) what remains of the bonus more than makes up for it.
For example, suppose you sign up at an on-line casino that offers you an additional $100 when you put your own $100 into your account there. In order for you to claim the bonus, the casino requires you to wager 5 times your original investment of $100. So, you play the smallest wager they allow until you've wagered $500. Once you've reached that milestone, the $100 bonus is yours to keep.
Now, suppose the casino's advantage in the game they offer is 2%. You wager $1 per hand for 500 hands to satisfy the bonus requirement. You lost on average two cents per hand you played (the 2% disadvantage). That means you'd lose approximately $10 of your original $100 investment, but you get to keep their $100. You've come out $90 ahead, and you close the account and move on to the next casino.
Apparently, there are still plenty of people who don't understand the power of bonus hustling. They just keep playing until all of their money (and the bonus) is gone. If more people understood as much as you now know about bonus hustling, the on-line casinos would have to stop offering them.
The example I gave is somewhat simplistic, but I think it explains the general concept. Norm Wattenberger (a frequent contributor here) has studied and played the on-line casinos extensively. Hopefully, he'll chime in.