When you've gone so long without washing underneath your foreskin, that a special rot starts to form, usually made up of different waste such as, mainly remnants of cum and sometimes even piss and dirt. This rot has been nicknamed "cheese". It is most often first formed below the head of the penis or the "neck" which is where the name is derived from.
Person 1: "yo dude i havent washed my dick in like, a week. Theres so much neck cheese now!"
Person 2: "Yeah i can smell that bro, pretty gross not gonna lie. Anyways, can i have taste?"