The ends of the bones are coated in a white layered structure called (articular) cartilage. This structure is highly specialised and creates an almost frictionless surface for the knee to glide against. Thus any injury to this smooth gliding surface alters these properties and will cause issues with the knee. Injuries to the cartilage can be caused by an acute sports injury or gradually as part of a ‘wear and tear’ (arthritic) process.
Fixed between the femur and the tibia you have confusingly more cartilage – often termed ‘footballers cartilage’. This actually is the meniscus and has multiple roles in the knee. You have two horse-shoe shaped pieces of them on either side of your knee. They fill and deepen the gap between the femur and the tibia. Their role is many-fold but essentially they can be seen as a method your knee uses to spread the forces passing through your knee over a larger area – like a shock absorber. So when the meniscus sustains a tear this can alter the forces going through your knee and will speed up the rate of wear and tear in your knee.