Subsribers see stories before everybody else. These stories are marked in red to indicate that they aren't published yet. However, the Slashdot interface doesn't tell you how long until they will be published. The stories are just marked as "To be published in the Mysterious Future!"
As a subsriber you also get a customized RSS feed. This feed has all the same stories you see on the home page including the stories soon to be published. Interestingly, the actually publish time for the story is included in the RSS feed. You can tell exactly when the story will go live on Slashdot. My RSS reader really unmasks the "Mysterious Future"