Adsense is a system that has grown up among people who run their own websites, and allows them to monetise their sites without really doing very much. It is a simple matter of signing up to use Adsense on your site and allowing Google to have access to your site for the placing of ads that are relevant to your content. Adsense will do the rest itself and give you the tools to make money from the searches and clicks of people who visit your site.
This all sounds very straightforward and there is no doubt that many people will be looking for the “catch”. There is not really a catch to be found, but the simple fact is that people will spend longer on, and be more likely to click advertising content from, a site that they find interesting. The job of any webmaster using Adsense to make a bit of extra money is to make their content worth reading – if you can’t do that then you need not expect to make any money.