Literally translated, it means “cheese and pepper” pasta.  Traditionally, that has included Pecorino (or Parmesan) for the cheese, and loads of freshly-cracked black peppercorns for the pepper.  But instead of just sprinkling those on top, the cheese and pepper magically come together with the starchy pasta water and some butter (while the pasta is cooking) to form the most luxurious cheese sauce.  It’s 4-ingredient magic.