CTC = Combat Table Coverage
The combat table is the different types of results that can occur on an attack. For a physical melee swing, the type of attack most often referred to from a tanking perspective you have the following results (as a defender).
I'll exclude the events of Glancing Blow, Crushing Blow, and Critical Strike for simplicity as these won't apply at level cap in PvE content for a properly speced tank (and aren't sitting on the job). Each of the events on the combat table reach a grand total of 100%. Every time the boss takes a swing at you there is 100% chance of one of these events occuring. However, the higher some events get, the later events end up being reduced as a result.
For simplicity sake, lets say you have 5% chance to be missed, 10% dodge, 10% parry, and 30% block... then the mob attacking you has a 45% chance of landing a normal hit. If you increase your block to 40%, the chance of a normal hit landing on you is reduced to 35%. Effectively by increasing the likelihood of a block occuring, you reduced the likelihood of the lower priority event (normal hit) from occuring.
This is where the concept of Combat Table Coverage (CTC) comes into play. By increasing one thing, you decrease the likelihood of another. Full CTC occurs when it is no longer possible for you to be hit by a normal hit (miss, dodge, parry, and block "covered" the full set of possibilities).
How do you maximize your CTC?
Mostly by increasing your chance to block. Dodge and Parry do contribute as well, however because Dodge and Parry suffer from deminishing returns (each point of Dodge Rating or Parry Rating is worth slightly less than the prior point). Block however doesn't have this bahavior and you can increase Block by increasing your Mastery and it continues to grow at the same rate until you reach full CTC (once Miss, Dodge, Parry, and Block total 100% of all attacks on you, getting more Block will have no effect).
Reforging and Gemming
Until you reach full CTC, Mastery is your highest valued stat as a Protection Paladin. Simply put, any piece of gear that doesn't have Mastery on it will likely have a stat reforged into Mastery. If Mastery is already on the item, then you would likely only reforge the item to minimize the effect of deminishing returns on Dodge and Parry (ideally you want Dodge and Parry to be extremely close because they both deminish at the same rate, if you have significantly more of one then you would get more total avoidance by swapping some of it to the other). Gemming will almost exclusively favor Mastery.
The "Real" Numbers
Sadly after explaining all of that, now I get to explain the number 102.4% which you may find referred to quite often. For every level an opponent is higher than yourself, you lose 0.2% chance of any of your defensive events from occuring. So if you are fighting a raid boss, the chance to be missed which is 5% normally is reduced to 4.4% because a raid boss is considered three levels higher than you. This same reduction of 0.6% happens to Dodge, Parry, and Block as well.
4 * 0.6% = 2.4%
Since you effectively lose 2.4% chance of your defensive events occuring (Miss, Dodge, Parry, and Block), you need to get 2.4% more of them to have full CTC against a raid boss. So when you total up your Miss, Dodge, Parry, and Block (when fully raid buffed) you are actually shooting for the number 102.4%.