These cyst-like growths are a symptom of Thargoid interference with a planet's ecology.
— In-Game Description
The pods are cyst-like growths at Thargoid Surface Sites. They are outside on the ground and on the ceiling inside the structure. There's a large quantity of pods inside the center of the structure. They're caused by Thargoid interference with a planet's ecology.
Parallels with Insect Eggs
Most insects start life as an independent egg. This reproduction method is called ovipary. A female insect produces the eggs. During mating the female insect receives sperm from a male partner. This can be stored in a special part of her reproductive system. When she lays eggs the sperm fertilizes and fuses with the egg nucleus. The embryo grows inside the egg while consuming the yolk. When the yolk is consumed the immature insect will chew its way out of the egg or the shell cracks open due to its size.
The Thargoid pod could be an organic vessel in which a Thargoid develops. It's not yet known what produces and lays these pods at the surface sites. The Scavengers appear to nourish the alien pods on the ground.