What does the Legendary ending mean?
The Master Chief and Cortana are floating, lost, in the outreaches of space. Years might pass before they are discovered again, so the Chief places himself in cryogenic sleep, asking Cortana to "wake me, when you need me." Fade to black.
Beat Halo 3 on the Legendary difficulty, however, and you'll see the following afterwards:
Our best guess: We have no way of telling when the above scene takes place. It could be mere moments, or long decades, since the Chief and Cortana said goodbye. For all we know, the two traveled back in time.
We can guess at where it takes place, though. It's definitely not another Halo, as the middle of the circle is not empty; you can make out a surface. In fact, based on the appearance of structures and lights on that surface, this object is probably a civilized planet of some sort.
Is it Onyx, the planet in the Halo novelization Ghosts of Onyx? Many fans say the shining symbols on the surface match those described in the book. If so, Master Chief is about to rejoin his family... the rest of the SPARTANs and their creator, Dr. Catherine Halsey, are currently trapped on the Forerunner world.
That's the popular theory, but it's also possible that Master Chief is about to discover a planet of still living Forerunners. Or that he's slept for centuries and arrived at a further advanced Earth. Or that he's wandered into the beginning of developer Bungie's other game trilogy, Marathon.
We won't find out until Halo 4. Which brings us to our last unanswered question...