It is a massive wooden door with gears that have dwarven symbols on them. In front of it you will find 4 pedestals for runes. In the middle of the landing with the pedestals you will find a symbol the Vault Hunter can later interact with.
It is locked by Dwarven Door-locking magic. You must collect 4 runes, as explained by Claptrap the Wizard, and then speak the phrase to open the door.