Okay, I tried finding a link for this to explain, but I couldn't so here goes freehand. ReBoot was a fan-loved animated series running from 1994 to 2001. Think of it as Tron being a TV series (except no real contact with the outside world and Jeff Bridges; although there is mention of "Users"). When "The User" loads a game, a Game Cube drops on a random location in Mainframe, sealing it off from the rest of the system and turning it into a "gamescape". Bob [the main "superhero" character] frequently enters the games, "Reboots" to become a game character, and fights the User's character to save the sector. If the User wins a game, the sector the Cube fell in is destroyed.
On June 1, 2008, it was announced that there will be a trilogy of ReBoot films coming to theaters. How I missed this, I have no idea. Don't get me wrong, I'm not a die-hard fan of the show (saw it back in the day, got too mature for me - I WAS LIKE 5 YEARS OLD) but if I saw it again, I might understand it, especially now that I've seen Tron and am currently awaiting Tron Legacy.
Here's the teaser. It doesn't do much other than show off the city and remind of the "Game Cubes".