It's a portal to the Sword of Akasha. That's where Nunnally was at the end of season 1, but she didn't know because she was blind/deaf.
Only Geass users can open the gate. V.V. made a contract with Charles so Charles could open up the portal and let both inside.
How V.V. got into the portal at the end of season 1 was because Rollo was with him, as in Lost Colours. Rollo watched Lelouch's original defeat.