What substance does Kiri use to help Spider breathe Pandora's air? The answer is mycelia.
Mycelia are the threadlike networks produced by fungi. In Fire and Ash, Kiri uses her connection to Eywa and Pandora’s living systems to infuse Spider’s body with mycelia after his breathing mask fails. The process creates a symbiotic biological change that lets him breathe Pandora’s atmosphere.
The scene is important because it changes the boundary between humans and Pandora. Spider does not simply borrow a mask or operate an avatar remotely; his own body is altered. Later, the RDA becomes interested in studying the change because Spider’s biology might potentially help other humans survive Pandora without breathing equipment.
The answer is not kelp, amrita or unobtanium. Kelp is associated with aquatic life around the Metkayina, amrita is a valuable substance harvested from tulkun, and unobtanium is the mineral that originally drove much of the human exploitation of Pandora. Mycelia specifically refers to the living fungal network involved in Spider’s transformation.