It means that's the big sea where all thoughts, all knowledge resides; all your dreams come from; all the things that you might not do in real life, in a dream, will come from. That's where it manifests from.
Inamaanisha hiyo ndiyo bahari kubwa ambayo fikira zote na maarifa yote yako: ndoto zako zote zinatoka, vitu zote ambayo hauwezi fanya kwa maisha kwa ndoto, zitatoka.