He means that the readership of HN is high value. Name recognition amongst the HN readership might reasonably be considered to increase your chances of getting a nicer job, a book deal, a Y Combinator spot, a freelance contract, an investment round etc etc.

What jacques_chester said, except I was mostly thinking about the engagement level of the traffic. I.e. I go to HN for deep reads and to find out about cool new tools and services. When I go to reddit, for example, I'm there to lol at teh funny .jpgs.