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.
All heroes that require Absorption also require the Compartmentalization Perk or the Lightweight Mastery to be used effectively.
Absorption has also been covered 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.
Note: The definition of a basic ability is an ability that does not require a Mastery to use. For example, Predator's blink ability is NOT a basic ability because it is only available if we pick the Blink Mastery.