What your looking for can be found here on Alla.
At the top there is a drop down bar, under game info, spells you will find priest. From there you can find bubble and look up what it does.
Bubble time If a priest casts that on you, it absorbs damage being dealt to you, so in theory thats a good thing. Why you've probably heard that priests should never bubble the tank, at least straight away, is because warrior's and druid's threat is built based on the amount of rage they have, and if they don't take any damage they can't build as much rage, hence can't build as much aggro, and from there everything just takes longer, or they might lose threat and the mob kills a DPSer.
This is not so much of a problem for a pally tank, but spiritual attunement gives mana back when you are healed, so you want to be taking damage as well.
In the end, as a pally it's probably not so much of a big deal, but have your priest save their mana for heals rather than bubble.
Edited, Nov 4th 2008 6:47am by slightlysober