Pipeline aims to help answer some of the questions that high school students may have if they're planning on entering the world of game development.
According to the official site, Pipeline will be created and maintained by high schoolers doing an internship with the company. The site will explore some of the more frequently asked questions about the industry, such as what to study, or what the best way is to find work with a games developer.
According to the site, Valve is also treating Pipeline as a bit of an experiment:
Traditionally, Valve has been a very good place for very experienced video-game developers, and not so good at teaching people straight out of school (the reasons for this and the trade-offs are covered in the Valve employee handbook). Pipeline is an experiment to see if we can take a group of high school students with minimal work experience and train them in the skills and methods necessary to be successful at a company like Valve.
At the moment, there's not much to the Pipeline site, but you can sign up for the newsletters and you can watch the attached video below.