This is a developmental feature, which is not yet released. These features might change or disappear during further development.
There are two variants: player-created absorbers, and scanner-created absorbers. Both appear as tall obelisks.
Player-created absorbers (yellow obelisks) cost 5 energy units to create, then 2 units per stage thereafter: each stage makes the absorber taller, and it takes a little time to grow.
Alternate plans:
Killing the aborber costs 5 units (yes, you read that right: costs), but thereafter it releases its energy as trees, in stages, and no longer absorbs; its death is a sure path. Absorbers are vulnerable for a short time at the beginning of a growth phase; if they are scanned, their death sequence is triggered.
Strategically, it offers a way to depose difficult Sentinels, especially when more than one is used (an absorber gains height more efficiently than boulders, but requires investment), or of moving trees. Absorbers may also create runaway scenarios, if they grow by absorbing trees.