This means that two points in Suppression will completely negate most of the miss chance of your Affliction spells against targets of your level and three points will completely negate most of their miss chance against targets up to two levels above you.
Note that this boost to hit rating will not be displayed on the warlock's character sheet. This means that a warlock with 3/3 Suppression only needs to target 14% spell hit through normal methods (gear, gems, other buffs).
To a Mage, a resist means he misses out on that damage. But since Affliction spells are mostly dots and mostly instant cast, a resist just means you cast it again. Suppression helps in these checks, meaning that Suppression indirectly increases your damage against anything that can resist many times, such as a higher-leveled mob.