With a casino bonus you may hear the word "wagering" thrown around. All this means is that you have to gamble a certain amount of bonus money in order to convert it into real cash.
For instance, if you have a casino bonus of 5.00 and your wagering requirement is 30x you'll have to bet 150 in order for the money to be withdraw-able. 5 x 30 is 150.
It's very possible to receive a free bonus (through perks) and exceed the wagering requirement while winning big.
Like if you receive a 5 dollar bonus and win 1000.00 while betting 0.40 cent spins, you've just made 800.00+ (after wagering) for free and can proceed to withdraw that money, no strings attached!
It's best to review the T.O.S on any casino site. Many places only allow smaller bets. You can't just wager 150.00 as a bet right off the bat. Cause that would be too easy. Usually it's 5.00 wagering or less.
It all depends on the online casinos rules.