The alt-right pipeline is a series of websites, videos, books, and other online materials that teach people about far-right political views and white nationalism. It includes websites like Breitbart News and Gab, as well as YouTube videos from certain alt-right personalities like Milo Yiannopoulos and Steven Bannon. It is a kind of pathway that people can follow online to learn about extreme right-wing ideas and ideologies, and it's been growing in recent years.