throat slime is the thick mucous that is produced when a throat is penetrated, along with other moisture from saliva, snot, drool, and a bit from tears, it gets generated when the throat is invaded and is thick, white and the perfect lubrication that lets a rigid cock fuck on waves of pleasure. Different subs produce different quantities of the mucous / slime and the quantity of tears, and nostril mucous / snot vary according to the action. The throat mucous is also generated to protect the throat surface from stomach acid in case it comes up. For the top, the slick super-lubrication provided by the throat slime is silky smooth and delicious to feel, and if the gagging action keeps up, then the throat muscles grip and massage the cock even if fucking motion is stopped and the cock is just kept down the hatch at full penetration. Hope that helps understand.