"I mostly just hurt people!" - Johner
That's what Warcraft gameplay is all about really. Find what your looking for, hit it till it stops moving, take whatever it had you want and move on. But you need to do something to make that happen. You have to hit your target.
The formula for hit rating changes with level. What that means is +1 hit rating to a level 1 is more valuable than +1 to an 85. It also makes discussing the whole thing a bit of a nightmare without graphs and flow charts. So I'll focus on the more practical stuff.
When you open your character sheet, ten hit the little arrow in the lower right labeled 'Show Character Information' you get all your combat stats. What were interested in right now are the melee, ranged, and spell sections since they all are things that can miss. For the most part only damage spells and attacks roll to hit, but that is not a hard and fast rule.
When you mouse over your hit chance it will show you your total hit rating, what % improvement that gives you, and below what your miss chance would be for things 3 levels above you. Since all bosses are 3 levels above you +3 is also also a /skull. Now leveling you want to fight things +1-2 levels to you for good exp, so those will be the numbers you focus on.
Now it gets more confusing depending on your class and even what specialization you chose. If you see 'Normal Attacks' and "Special Attacks" the normal attacks are you just flailing about with your crowbar and the 'special' attacks are things you hit buttons for. usually. The only thing consistent is there's an exception to the rules :)
If you use a two handed weapon, or two one handed weapons your accuracy will go down. They hit harder, but less often. It works out so you get an advantage but not a huge one. Dranei gain a racial advantage of +1% to hit, so their base miss is 4 not 5 percent.
When leveling you will come across items with +hit, and have access to some enchantments that do the same.
So we know hit rating is good to grab when leveling, but itsn't usually vital. But when you hit 85 it's vital right? Well, its another one fo those yes and no answers.
You need to raise hit rating till you max out how much you need for the content your doing. This is where sites like AskMrRobot and others fail. They assume your going to be facing boss level mobs (level 88) right away so push you to dump more into plus hit than you really need. A bit of thought would do you much better.
In Grim Batol the boss is only 87. So if your dumping points into getting hit rating up for a level 88 your wasting them. Get your hit rating to 0% miss for 87 and after that you can work on crit, haste, etc. It makes running heroics much smoother when you can hit harder and faster by not wasting the points on extra hit rating you don't use. It can be a huge difference in damage.
But what if you occasionally hit a random raid, like the ones in Baradin Hold? Your going to miss right? Is it time to be scared?
Actually no. You can take a trip to the Arcane Reforger and in a few minutes bring that 88 hit ratingto 0 with little fuss or muss and just a little gold. You will lose a bit of DPS - other stats will be lower after all. But you should be able to pull through the fight with no trouble. Once you start raiding often you should probably just shoot for zero miss at the level 88 mark and leave it unless you have a pile of extra gold you don't mind using.