From the side one can appreciate the incongruity of the rounded lower end of the femur and the flattened upper end of the tibia.
The two menisci help to reduce this incongruity and absorb shock between the bones.
PF and TF joints
You can see also that there are several joints - bony points of contact - not just one. These joints are marked on the small X-ray.
The position of the patella relative to the groove of the femur changes as the knee is bent. In the straight knee the patella lies above the groove and can be freely wiggled from side to side. As the knee is bent the patella engages with the groove of the femur and it should not be possible to wiggle it from side to side.
When you look at the X-ray again, you will see that the patella bone seems to be 'floating' - but it is just that the tendons above and below it do not show on X-ray. Neither do the two shock-absorber menisci, which appear just as a dark gap on X-ray between the two bigger bones. [We also omitted from this little series the quads muscle - normally attached to the top of the patella - just to better illustrate our point].