When you hover your mouse over certain items (other characters, NPCs, mobs, items, buttons, etc.) a small window will pop up displaying a description of the item. This also works in many other computer programs. For example, hold your mouse over the "W" logo image ( ← this one should show "This is a tooltip") in the upper-left, and a small box saying "Wowpedia" will appear.
This term comes from general computer UI tool tips from buttons or icons.
See Help:Tooltips for information about how to make tooltips work on special links in Wowpedia.
Mousing over the Barracks building icon in the Battle for Stromgarde before it's constructed
Mousing over the Tree of Life on the Battle for Darkshore map