Slop is a food eaten by Have-Nots. Not much is known about it, other than that it tastes horrible. There are many ways to avoid Slop: being the Head of Household, winning the Have/Have Not Competition, or receiving a slop pass.
In Big Brother All-Stars, Slop was introduced as a replacement for Peanut Butter and Jelly Sandwiches, which were formerly eaten by HouseGuests who lost the food challenge. Slop has appeared in every season since.
It has been speculated that Slop is really Bill Pearl's High Protein Oatmeal - Unsweetened, which has many of the essential nutrients, vitamins, etc. to maintain a healthy diet for the HouseGuests.
It's also been speculated that Slop is a Soy-lent mixture, also containing necessary nutrients for a healthy diet.