It refers to a platform on which ads are run and managed online.

Google Adsense is an application run to serve the ads posted in the internet. People owning websites can always enroll in these programs to enable images, texts and even video based advertisements get displayed on them. Being designed and developed by Google, all the advertisements are under their observation. The ads are supposed to generate revenues through per-impression or per-click basis. Adsense has become really popular with website owners because it is a less intrusive way of displaying ads online at least as compared to most banners. If a website has great content in it, then Adsense can be used to monetize it and with great effect.