The dark web is made up of sites which cannot be accessed and searched by using conventional internet browsers. To enter it you generally must use the TOR browser, which bounces through servers all over the world in random patterns. It was originally developed in response to needs of users located in oppressive countries, such as China and North Korea, that block access by their citizens to websites they consider to be dangerous and subversive to the ruling regime. Most of the dark web contents were, and still are, those serving such oppressed users. But it has been co-opted by more sinister forces with different agendas.
Googling TOR will tell you everything you might want to know about the browser and how it is used to access the dark web.