It's a calculator for Power Word: Shield. The simplest way to use it is to plug in your spell power and hit calculate. You can also add a “booster,” which is used to mimic the T10 set bonus (5%) and/or the ICC zonewide bonus (5–30%).
(Until I see math indicating otherwise, I'll assume that those bonuses are applied after the normal shield value is calculated. Please link if you've done or seen this math!)
You can also compare two shields. E.g., how much of a boost would your shields get from an additional 100 spell power? The comparison shows both hit points and percentages.
The formula and some useful resources are below.
Here is the formula. For simplicity, I've assumed you're fully talented into all relevant talents.
PWS = (Base+(SP*(coeff+BT)))*(1+TD)*(1+FP)*(1+IPWS)
Base = 2230 for max rank
SP = your spell power
Coeff = 0.8068 (the spell power coefficient)
BT = 0.40 (5/5 Borrowed Time)
TD = 0.05 (5/5 Twin Disciplines)
FP = 0.04 (2/2 Focused Power)
IPWS = 0.15 (3/3 Improved PW:Shield)
Now, whether or not the formula is working as intended is an open question. The comments and bug reports on the WowWiki page are very interesting. That page also lists some tips for using PWS, as well as the history of PWS in patch notes. Highly recommended reading.
Also, Zusterke has a full-featured priest spell calculator. It does much more than this page ever will, including the ability to take less than full ranks of the relevant talents. It also does all priest spells, and works for (gasp!) holy priests too. It's a fantastic resource.