What does this do?
It adds an item's ItemLevel to its tooltip.
What's an ItemLevel?
Quoting WoWWiki:
"The item level is a rather important property of every item. It has two main functions - reflect the items usefulness and at the same time determine the minimum level a character must have in order to use it (see Minimum level requirements and Item Level). Unfortunately the true item level is hidden in the game. Only UI mods can read it via the programmers interface."
With this mod, you can behold the hidden true item level. Muwhahah... err. Yeah.