The deep web is online content that is not calatogued or indexed in the usual search engines. It contains 7500 TB of data as opposed to the regular web, also called surface web, that contains only 19 TB of data. The deep web, also called invisible web, dark web, undernet, contains content that you wouldn’t want to watch, but also is the underlying structure for a lot of drug trafficking, illegal activities and more, some of them paid for with Bitcoins with the help of a black market called Silk Road 2.0.
A piece of advice: if you have a friend that tells you to go in the deep web, please don’t. The web is as safe as you make it to be and the danger lurks only one click away, so be wary.