You must find 5 books that are scattered around Dream World. Some of these books are a few floors deep with dangerous monsters so it is not recommended to complete this quest until you are level 200+.
Once you've read the 5 books you will be able to speak with all the NPCs in Dream.
“You have read all 5 books about Dream Speak. The people in town will understand you now.”
Go back to Quests.