PFPS is an abbreviation of 'Patellofemoral pain syndrome'.
A syndrome is a collection of symptoms that tend to go together. It is not indicative necessarily of the cause of the symptoms. The descriptor tends to be applied to people with pain in the front of the knee, especially young women often with knees in dynamic valgus which can be corrected with attention to hip abduction exercises and footwear to correct rear-foot eversion and flat feet.
Patellofemoral pain syndrome. Petersen W, Ellermann A, Gösele-Koppenburg A, Best R, Rembitzki IV, Brüggemann G-P and Liebau C. Knee Surg Sports Traumatol Arthrosc. 2014; 22(10): 2264–2274.