Avoidance refers to a player's ability to nullify certain attacks. For melee, dodge, parry, and miss are all different ways to avoid the attack. For spells, resistance can raise your chance to completely avoid an attack. Bubbles from items and abilities, such as Power Word: Shield, use absorbtion and if strong enough, can completely avoid an attack. There are bubbles that avoid both spell and melee damage or only one type.
Avoidance should not be confused with mitigation, which is the partial reduction of an attack. Block and armor for example can never give full avoidance, but can give very significant mitigation and help avoid white swings.
It is when your cumulative Dodge, Parry and Block + 5% Boss miss equal or exceed 102.4%. You are now un-hittable. You will at least block a melee attack, and can avoid all white attacks.
The extra 2.4% may come from the fact that Dodge and Parry are subject to diminishing returns. For Warriors, Shield Block does allow you to add 25% to your block number above while it is active. Trinket procs are also added in to see if you are above the avoidance cap. The extra 2.4% is also believed to come from fighting higher level bosses. Since a raid boss is higher level than the tank, it requires slightly more avoidance to completely avoid all white hits.
For Warrior tanks, being over the avoidance cap with the use of Shield Block means the amount be which you exceed the 102.4% is added to your critical block rate. This does not apply to regular Block rating. Beyond 102.4%, mastery only grants the critical block, not the block %.