Spell Absorption is a spell of the Mysticism school of magic and will absorb an attacking spell as an increase in magicka (proportional to the original spell's power). The magnitude is the chance that a spell will be absorbed and the duration is how long the spell will last. The caster's magicka pool cannot be raised above its maximum value by Spell Absorption.
Spell Absorption can be useful after summoning a creature with a magical attack (e.g. Scamp). Attacking the summoned creature will turn the creature against you, thus forcing it to use magical attacks against your Spell Absorption and, theoretically, resulting in a net magicka gain. However, it is difficult to make a profitable amount of magicka unless you have the birthsign of The Atronach.
It is important to note that Blessings from shrines can also be absorbed as magicka.