In the context of stats, the term "cap" refers to the point at which equipping more of a particular stat will have no additional value. For hit rating the cap usually discussed is in regards to raid bosses. For example, once you reach or surpass the hit cap, you will never miss an attack against a raid boss unless you are under the effect of some debuff that reduces your hit chance (for example, Attumen the Huntsman's curse).
As shown above, your base chance to miss a raid boss while dual wielding is 27% with auto-attack on both hands. Your base chance to miss a special attack is 9%. For each 15.77 hit rating at level 70 or 32.79 hit rating at level 80, you reduce your chance to miss by 1%. Thus we can calculate hit caps for auto-attack and special attacks:
A "soft" cap is the point at which your yellow attacks will always hit and more hit rating has no affect on them. A "hard" cap is the point at which your autoattacks will always hit and more hit rating does absolutely nothing for you.
All melee in level 70:
* Dual-wield auto-attack: 426 hit rating
* Special attack or Single-wield auto-attack: 126 hit rating
Fuente
Hit - WoWWiki - Your guide to the World of Warcraft