There are many theories on what ectoplasm is, each as disturbing as the last. Some say that it’s emotion, condensed over years of existence. Others think that it’s some sort of infection of the soul, as only the undead seem to produce it.
In truth, ectoplasm is none of these. It’s the outer shell of a soul, the skin more or less, and acts as a sort of lubrication between the flesh and the spirit. Tiny bits of reality combine with the immaterial otherness of the underworld and congeal into a sort of spiritual mixture that ghosts and other undead find incredible useful.
Necromancers prize the stuff, finding a myriad of uses for it. Most people find it weird and decidedly “icky”.