The higher your hit rating is the less likely you are to miss against mobs. You will understand hit rating fast if you have ever been in PvP and stood there casting Aimed Shot and then it missed. Then your target runs up to you laughing in your face.
If you are below your hit rating then you will miss against mobs that are lvl 83 (Bosses) when you are lvl 80. If you have a higher hit rating than below then that extra hit rating is being wasted. Once you get into good gear and you are over hit capped then look into what gems and enchants you have that increase hit rating and change them out to Agi or Critical or Attack Power gems/enchants.
There is a talent low in the MM tree called Focused Aim. This will boost up your hit rating.
With no points in Focused Aim you need 296 hit rating to be capped.
1/3 Focused Aim: 263
2/3 Focused Aim: 229
3/3 Focused Aim: 197
If you are a Draenei or have one in your group, you need 263 hit without Focused Aim or 164 with 3/3 Focused Aim. That is if YOU have those talents, not whether the Draenei has it.