Enmity is rife in the Warhammer world, where many races have nurtured grudges and animosities against others for thousands of years, and overwhelming hatred is a potent force in battle.
Source: Warhammer Fantasy: 8th Edition
A model striking a hated foe in close combat re-rolls all misses during the first round of combat – this represents the unit venting its pent up hatred upon the foe. After this initial blood-mad hacking, the impetus is considered to be spent – the rest of the combat is fought normally. Sometimes a model will only Hate a specific foe (rather than everyone). Where this is the case, the type of foe will be expressed in the special rule, for example Hatred (Dwarfs).