There are three known means of reducing Corruption; Grief Seeds have a capacity to reduce Corruption when applied to a Soul Gem, and POTION OF MANA reduces Corruption before replenishing MP. It is also possible to use MP to reduce Corruption on a one-for-one basis via MEDITATION, though only Homura is known to possess MP.
Corruption is a measurement of the corruption accumulated in a Soul Gem. When the Soul Gem is fully corrupted, Corruption is at maximum. Corruption can be reduced by using items or skills. Maximum corruption is dependent on LEVEL.