Why does gulliver travel to glubbdubdrib?

Gulliver can't immediately find a ship to take him from Balnibarbi to the island of Luggnagg. He's lucky, though: he makes two random friends (whose names we never even learn) in the port city of Maldonada. They offer to escort him to a nearby island for some quality touristing. This island is called Glubbdubdrib, and it's kind of a secret island.

What do you mean? I don't understand