Forgive the noobish question but I've been seeing that here since I've returned to the game and was wondering what it is.

Mana Per 5 seconds.... So, +10 MP5, means the item will cause 10 mana per 5 seconds to be regenerated during combat (aka within the 5SR [5 second rule]).

Here's the geeky complete answer. (http://www.wowwiki.com/MP5)

Thanks for that information. I've gotten some gear with that on it. I'm not sure how much it would benefit me because I'm a resto druid and usually focus on getting +heal and int stuff. Guess I'll search through the older threads here and see how this ranks up there.

As a resto Druid MP5 is one of your primary stats (in no particular order, +int, +spr, +heal, +mp5). The more you have the longer you can last during boss fights without having to use innervate and/or mana pots.

In short +1 int < 1MP5 and +1 heal <<< 1MP5 and +1 spr << 1MP5. So an item with +10 int vs one with +10 MP5, I'd take the +10 MP5. Of course this depends on your talent spec, what you are doing (raid v 5-man v PvP) and the relationship between the stats. Ex. +1000 healing does you no good if you only have 3 int. ;D

For the exact ratios, read http://www.resto4life.com/ Great blog. In particular read the "VS Series" entries in this post (about half-way down), http://www.resto4life.com/2008/01/03/resto4life-or-at-least-2007/. Don't get hung-up with the math and/or charts (unless you want to). Read the supporting text and follow-up comments.