Although blood is mainly a liquid (called plasma), it also contains small solid components (red cells, white cells, and platelets). The platelets are best known for their importance in clotting blood. However, platelets also contain hundreds of proteins called growth factors which are very important in the healing of injuries.
PRP is plasma with many more platelets than what is typically found in blood. The concentration of platelets and, thereby, the concentration of growth factors can be 5 to 10 times greater (or richer) than usual.