There are companies that offer pre-written, or written-to-order, university assignments. Often they employ recent university graduates to write papers that current students can then pay for and submit as if it's their own work.

One meaning of the word "mill" is a company or institution that produces something mechanically, rapidly, for the most profit, without attention to quality. A "puppy mill" is a disreputable breeder who raises puppies (young dogs) under bad conditions. A "diploma mill" is essentially a fake university that issue diplomas to anybody willing to pay, without really teaching anything or testing. A "LASIK mill" is a medical practice that is set up to do lots of LASIK operations very quickly, in order to make the most profit. A "paper mill" is a place that sells term papers that you can copy and hand in, without doing the work yourself. In addition to being dishonest and against the rules of any university, using a "paper mill" is risky, as universities have automated tools with huge databases of term papers in them, and are very good at detecting plagiarism (copied work). Of course, the original meaning of "paper mill" is a factory that processes wood into paper.