Absorption is a perk that is selectable from the beginning of the game. When using the perk, any time you use a basic ability, it will permanently add maximum energy and energy restoration to your hero. This effect will stack up to 100 times. This perk is great for those who constantly need energy for their abilities where their cooldowns are up long before the energy is available.
Absorption is a great pair with the perk Compartmentalization.
This perk is mostly only chosen by Nova or Tassadar, as the other heroes will easily have enough regen, and there's far better perks for them. The great thing about chosing this perk, is that you can skip Energy Mastery and Talents if it was in your build, efficiently freeing up a mastery for other stuff. There are also certain more exotic builds which require a lot of Energy, for example, a support Anvil with LW will need Absorption in order to use all of his skills actively.