Hacker News is a news aggregation website created by Y Combinator where users can register an account and participate in posts. Posts will typically contain a link, which is usually a project or an article to read. Users can "upvote" the post which will (with enough upvotes) send it to the front page of the website.
There are a few different sections of the website, the front page which holds the best posts, the new section which has the newest posts and links and the Ask/Show HN sections for showing Hacker News members projects or asking them questions.