[@ERode]
Poison Attack at Rank I allows certain natural attacks--claws, fangs, etc--to produce a poison. It can be one of two possible weakened forms compared to creatures who would "naturally" have their own venom: A weaker version of Paralysis, which essentially Slows a target, or a weak Damage over Time. Increasing the rank can raise the potency, the chance of infliction, and so forth.

Muffle boosts stealth by quieting the sounds you make to a certain degree. Increasing the rank is pretty much a straight upgrade to the effect as it applies to clothes, environment, etc.