SPP is an abbreviation of 'suprapatellar plica'.
Significance of a suprapatellar plica
The structure is a natural anatomical structure but is not found in every person. Mostly it is flexible and causes no problems when the knee bends and stretches, but sometimes it can be injured and become swollen and thickened and catch between the femur and the quads tendon.
The Role of Complete Suprapatellar Plicae. Mine T, Chagawa K, Ihara K, Kawamura H, Kuriyama R and Date R. Arthrosc Tech. 2016 Feb; 5(1): e197–e199.