Toughness is a tank stat, it reduces the damage bonus an attack has when it critically strikes. For example when an Expert 1 boss crits they deal an additional 50% damage. Toughness offsets the crit bonus at a 1:1 ratio i.e. 1 Toughness = 1% reduction in the mobs crit bonus. To completely offset the crit bonus of an Expert 1 boss the tank would need 50 toughness.
Focus is a DPS caster stat, it offsets a mobs chance to resist a spell attack and also the amount of damage reduced by the mobs resistances.
Hit is a DPS melee/ranged stat, it offsets a mobs chance to dodge and parry an attack as well as a characters chance inherit chance to miss.
Below are the various Hit, Focus and Toughness rating goals for various content tiers.
Level 50 and below dungeons – No hit, focus or toughness required
Expert 1 Dungeons (Realm of the Fae, Iron Tomb, Foul Cascade, King’s Breach, Fall of Lantern Hook) – No Hit, no Focus, 50 Toughness
Expert 2 Dungeons (Darkening Deeps, Deepstike Mines, Runic Decent, Abyssal Precipice, Chamer’s Caldera) – 100 Hit, 100 Focus, 100 Toughness
10 Man Rift Raids – 100 Hit, 100 Focus, 100 Toughness
20 Man Instance Raids – 200 Hit, 200 Focus, 150 Toughness