In World of Warcraft, flavor text is an additional piece of text that makes a comment about the piece of gear or item that it is on. Not all gear or items have flavor text on them.
Just one example of flavor text is:
"A grotesque skull now rests atop the blade. Etched in blood across the axe head is one word repeated ten times: SUFFER."
This flavor text appears on the epic pole arm Oathbinder, Charge of the Ranger-General (linked below).