Online casino bonuses are simply used by an online casino to entice the player to make a deposit at their casino online to play the games the casino hosts.  The online casino bonus is stated in advance of the deposit and usually has a percentage it is worth, as well as a ceiling on how much money one can get out of the bonus offer if they choose to maximize it.  For example, an online casino bonus may look like the following:

100% up to $200 instant first time deposit bonus.

The online casino in this example will instantly credit any new first time depositing player with up to $200 bonus money when they make their first time deposit.  If the player deposits $100, then the casino matches the deposit.  If the player deposits the full amount, $200, then the online casino credits the player with an additional $200 in their online casino player account.  

The amount credited in bonus money is in addition to the amount that the online casino player deposits.  This additional money may be used by the player to take part in most of the games that are made available by the casino online.  While the player can use the money for play, they often may not withdrawal or cash out the money from the casino bonus straight away.  Instead, the player must work off or play through the bonus money a certain amount of times before they can cash it out.  For example, if the player receives an online casino bonus of $200, he may have to play 20 times the received amount in online casino games before the money is his free and clear for cash out.  When used smartly and prudently, this bonus money can really help the bankroll.