Well, they describe the angle made by the knee joint.
Varus or 'Genu Varum' mean the lower part of the leg angles towards the midline.
This would make you bow-legged
Valgus or 'Genu Valgum' means the lower part of the leg angles out to the side.
This would make you knock-kneed.
Surgeons estimate these angles so the can keep track of the progression of the arthritis.
So now you know