What Is Adsense?
Adsense is an Advertisement system run by Google, if you have a website you can submit it to Adsense and upon approval you will be given an account where you can create your own AD banners with the size you choose and place them across your website.
Whenever a visitor clicks on any of your banners you get paid a portion of the revenue.
Adsense is the best and biggest Ad Network on the net since it's run by the giant search engine Google. This is why it is difficult to get your website accepted, I suggest getting some traffic first for a little bit before submitting your site/blog for Adsense approval.