A low-protein diet is a diet in which people reduce their intake of protein. A low-protein diet is often prescribed to people with kidney or liver disease. Also for those with inherited metabolic disorders, such as PKU.
Foods to avoid would be any animal byproduct such as meats, eggs, fish, poultry, milk, yogurt, cheese, and even peanut butter. Also bread, cakes, pasta, quorn, and nuts. It is surprising how much protein is in everyday foods.