Focus

Focus reduces the chance to miss with magical based attacks (Such as for clerics and mages). The needed amount varies between tiers. The chance to hit increases by .2% per 1 focus.

Focus Cap
In terms of the "cap" of stats, it refers to the point at which stacking more of that particular stat is not advantageous. This is because once you have reached the cap, going above is just wasted stats that are not required by your character to do anything else. In context to the focus cap once you hit the focus cap for you particular tier you will never miss an attack (unless you are under some sort of debuff that forces you below the cap). These numbers may need more number but they are good to shoot for when gearing up.