DAKLOL is right.
It's a complete minefield but essentially you get paid per click, on the adds that is....sounds simple but there are many criterior that have to be met before a 'click' is deemed valid....you'd probably only get 1 'valid' one out 100. Take into account that that 1 'valid' one is basically not worth the screen it's printed on and you can see that you would need many thousands of 'clicks' to make it pay anything useful.
Great if you're "Google", not so great for the individual. There is of course no harm in displaying adds but is it worth it? Could the space be used for something more productive?
Regards.