Notifies BG > Raid > Party > Say that you are close to run out of mana. It's some sort of healer protection against for example tank pulling mobs when you are out of mana.
You can easily set trigger value at which message will be sent. To change that go to Custom Options in WeakAuras addon.
The aura is triggered when your mana percentage is below trigger value. The aura is reset when your mana percentage is equal to or above trigger value + 10.
The aura is enabled by default only inside instances (player in normal / heroic instance).
Designed and checked on WoW TBC. Should work on retail too.