Flavor text is the text displayed in orange on some item tooltips, in quotes. It is usually humorous. For example, the flavor text for the references the kobold catchphrase "You no take candle!", and the flavor text for the references a real-world meme that sprung from a Thottbot comment on that item. Rarely, ability buffs have flavor text, such as the buff of Amorphic Cognitor.
Many interactable NPCs, such as vendors, have flavor or gossip text in their dialogue window that allows the player to interact with them. In some cases NPCs may have holidays specific lines. A large amount of these flavor lines were set to be automatically hidden with Wrath of the Lich King. As an example, the backstory of Galgrom and the text of Tradesman Portanuus on the alternate Draenor will be hidden by default. Hidden gossip lines can be seen by entering the following command in the chat window:
/run ForceGossip = function() return true end
Changes made have caused the previous line to become obsolete. This new line will do the trick post-Shadowlands:
/run C_GossipInfo.ForceGossip = function() return true end