Increases your chance to Hit with melee weapons and spells. To reach any of these caps, you just need to gain more Hit Rating. There are three caps you should know about: Special Abilities Hit Cap (8% Melee), Spell Hit Cap (17% Spell), Auto Attack Hit Cap (27% Melee). Hit Rating is the most effective secondary stat up until you've reached 17% Spell Hit, and at that point, you'll already have 8% Melee Hit.
The Expertise Soft-Cap of 26 will eliminate your attacks being dodged from behind. Having this cap and always attacking behind your target is crucial for avoiding high DPS loss. To check your expertise, open your character panel and then check under the Melee tab for "Expertise." Make sure it is at exactly 26 for both weapons. This also means that any Expertise beyond the amount of 26 is no longer very effective