Stage Pass is essentially a Fast Pass of sorts for the smaller venues, Stage 23 and Stage 28. The Things to Know section of the website says to just check back for more information, but in 2013 at a certain point in the day you could line up and get tickets for the smaller panels, on a timed basis...first come, first served until they filled. I don't remember the exact timing, but it was generally this: you could get passes for the smaller panels between X and X starting at a specific time, then later you could get passes for panels between Y and Y. I can't remember the other limitations (like I think you could only hold one Stage Pass), but I believe there were limitations.
I think it was 12pm or 12:30pm for the afternoon panels, I left the Animation panel in the Arena early to secure an afternoon Stage Pass - you can pick it up for others in your party, so only one of you needs to present all the badges.