This is the crossing from the continent of Angara, where the entire game has so far taken place, to the southern continent of Gondowan.
Much like the Kolima Bridge, this is just a drawbridge across a river, manned by guards from Tolbi. Iodem is waiting for you here. When you talk to him, he will "join" your party, in the same way as Kraden did way back in Sol Sanctum: He is traveling with you, but does not fight. If you try to go back to Angara with him, he will leave and wait at the passage. Otherwise, he will stay with you until the end.
Head East from Tolbi and the Passage to reach Gondowan Cave. This is an alternative route to return to Angara, since you can't take a boat back across the Karagol.
Upon entering, head up the left path and through a door. Use Lift on the boulder that is in the way, then continue down the path, entering a door facing north. Keep walking north on the path until you see several platforms in the water. Jump right to reach a treasure chest containing a Lucky Medal, then keep jumping right and finally north. Be sure to go back and get the chest that was on the northern path, because it contains an Apple. Go through the door on the right heading south, then climb up the stairs. On this upper level, use Move to push a stump out of your way, into the water. This will simplify any extra voyages through the caves you may make. Continue heading right until seeing a large stump, which needs to be Moved twice. Now, just walk south and out the door to get back to Angara.