Google Adsense is an online Advertising network that allows publishers to sign up with them and if approved you can place an ad code or codes on your website. When someone either views the or they click on the ad you get paid a percentage of what the advertisers pays google through adwords.
To earn money through Adsense you need good quality website traffic. The amount you earn will depend on how many website visitors you have to your website and what the cpc cost per click the advertiser pays for the ad.
Some ads only pay 5 cents while some can pay as high as $5.00 or more.
Adsense is NOT a get rich quick scheme. It takes lots of work and plenty of patience to start seeing money from Adsense.
I wouldn't quit your day job for adsense just yet. For a year or more you might only see a few dollars in your Adsense account depending on how hard you work on your website.