Google Adsense is an offering from Google which allows website administrators to add relevent advertising to their site quickly and cheaply. There is no need for your business to market for advertisers, Google does this for you.
A quick overview of the process follows.

You have to have a Google Account. Type in www.google.com, hit enter; the page that comes back will display navigation options below the search box when you move your mouse over the page.

If you are just starting with websites, there are several ways of making some money to offset the overheads of running a site. This includes, but is not limited to, Google Adsense; Amazon Associates and various Affiliate schemes.

Google Adsense gives you the opportunity to add small adverts related to the subjects on your site pages. You get paid every time someone clicks on the adverts.

Amazon allows you to put links into their sales pages either in the text or as small adverts; you make money when someone goes to the site and buys something.