I just created a troll priest and have been reading the forum and the 5 second rule is metioned. What is the 5 second rule

you don't regen any mana for 5 seconds after casting a spell

This is not correct.  Every class has two mana regen rates, one is while casting and the other is while not casting.  You can see the not-casting one on your spell character sheet directly, you will have to mouse-over that number to see the while casting number.  Blizzard defines casting as having used an ability that uses mana within the previous 5 seconds.

What does this mean?  It means, the two rates have nothing to do with whether or not you're in combat, as you can avoid casting for a period of time in combat, and you can cast spells out of combat.  It ALSO means that you can manipulate this rule to maintain the non-casting (and usually much higher) regen rate by utilizing certain abilities.  This also means that you do not re-enter the while casting regen rate until the spell actually goes off; IOW, the casting of the spell doesn't count toward the while-casting rate (confusing, I know).

Most classes, don't really care about this rule, because there's generally little difference between the casting and non-casting regen rates and their classes are built around constantly casting.  In fact, it only has any real effect on three classes (Priests, Druids, and Mages) and is really only worth giving any real concern to by Priests because Priests actually have talents, abilities, and items specifically geared toward manipulating this rule.

So you may ask, "How do Priests utilize this knowledge?"  The simple and obvious answer is with a technique I refer to as cast/canceling.  If you're single-target healing, pre-cast GH on your target, and let it cast for about 1.9-2.0 seconds (possibly a little more if your lag is really low).  If your target doesn't need the healing, cancel your cast (always with a /stopcasting macro, never with jumping, moving, or escape because the first two have a delay, and the latter could have negative effects), and immediately begin casting again.  In many boss fights, particularly where you have Paladins and/or Druids helping heal, you can often go several repetitions of this before the GH needs to go off.  If you happen to get at least two of them, you'll get at least one tick of OO5SR regen, which, for me, is worth about 250 mana.

But that's not the only way, as I said, Priests have abilities that specifically allow them to manipulate this, and the primary ones are Inner Focus and Holy Concentration.  Beside the obvious effect that both save you the mana cost of whatever spell you use, because they make the spell cost zero mana, they don't put you back I5SR.  Thus, if you're currently OO5SR, you can click IF and begin Cast/Cancelling GH so you don't lose ANY healing (since you're still going to have to do that), but your next GH won't put you back in, and you can get a good 5-10s usually of full OO5SR regen.  Holy Concentration works similarly, but it's a proc rather than a use, so you have to keep an eye out for it.

Priests also have some items that all them to make good use of this.  One of them is the Earring of Soulful Meditation.  If you see HC proc, or you know you'll get a good length of time to Cast/Cancel with IF, you can use that and, if you get the full 20s, you can get almost 1500 mana more every 2 minutes.  Bangle of Endless Blessings has a similar use, but it's only 130 Spirit instead of 300 Spirit, but it does have a decent proc

Another useful item for CoH priests is the Eye of Gruul.  It has a low proc rate and only reduces cost by 450, BUT it has a chance to proc off of each target from CoH AND CoH is less than 450 mana (405 IIRC).  IOW, it turns into about a 10% proc rate (more if the party has pets and essentially becomes another HC if you're spamming CoH.

As a Holy Priest, mastering the 5SR is imperative.  In fact, with the buff to Spirit in 2.4, with just some basic manipulation (like timeing my IF and ESM with HC procs), I almost never need to use mana potions if I have a mana battery (Shaman or Shadow Priest), and only have to occassionally when I don't, compared to when I didn't have it mastered pre-2.4 and often needed pots and sometimes an Innervate just to make it through a normal boss fight.