Misc Can anyone pls explain bonuses to me

Guys, as I said I am new to the world of online casinos. So I still haven't decided on a particular website and I am careful because if I create an account and don't like it I'll move to the next one and so on and start making deposits and it will become a problem at some point. So I started researching and I find it so overwhelming that I don't even understand some casinos bonus practice. Can anyone explain to me the what does this mean for example:
" But how does it work? Say for example, a casino is offering a 100% online casino welcome bonus, and you decide to deposit $100. The casino will add $100 to your account in the form of a bonus. All sounds pretty simple, but you will need to take into consideration some often very hefty terms and conditions. Wagering requirements nearly always form a large portion of the requirements and have to be complied with before any withdrawal can be made. "

 

There are many bonuses and bonus types. Good explanation here:

http://www.thegoodgamblingguide.co.uk/casino/here_are_the_best_casino_bonuses_explained.htm