With absorption any time you use a basic ability, it will permanently add +2.0 maximum energy and and +0.0195 energy restoration to your hero. This effect will stack up to 100 times. After absorption has been fully stacked, you gain +200 maximum energy and +1.95 energy regeneration.
Absorption is a great pair with the perk Compartmentalization.
Absorption has also been convered in the 1st part of the official Perk Detail Overview on BlacKcuD's blog. Check it out for more details on how it works, known exceptions and use cases.