This is a textual inversion embedding to add makeup/mascara running from the eyes, as often seen in fellatio/irrumatio images, but can be used elsewhere too. Use corneo_runny_makeup to activate it.
It can come out pretty heavy by default (i.e. a LOT of black mascara running down the face quite thick) so you may want to tone it down by de-emphasising the tag such as (corneo_runny_makeup:0.9). Also as with some of my other embeddings, the eyes themselves sometimes need some work in image gens that use it, so add some other tags to prompt how you want the eyes to look, and if that fails, try inpainting.