A Snot Rock is a rock of mucus located in either the mouth or the throat. It is usually formed overnight when sitting mucus begins to harden. It is very disgusting and I myself have only witnessed it one time.
So I woke up this morning and as I'm about to get in the shower, I feel the need to cough. After I cough I feel a lump in my mouth and so I spit it out and a Snot Rock comes flying out.