The word 'genu' means 'knee'. Genu varum means 'bow knees' or 'bow-legs'.
Varus means the same but is an adjective, eg varus deformity, varus force.
Bow legs are common in toddlers before the age of 4 years. Most kids simply grow out of it and no treatment is usually necessary.
Bowing may develop later in life, as arthritis breaks down the inner (medial) aspect of the knee joint, often due to a meniscus having been removed some years earlier ('medial compartment' arthritis).
In countries where diet and health may be poor, bow legs may also occur from such things as scurvy.