A flat foot is when the bones of the feet collapse downward with a step of the foot. The naturally-occurring arches in the feet are not supporting the foot adequately. Very often flat feet is an inherited condition but it can also be a result of medical or biomechanical issues.
Flat feet can often be a directresult of wearing improperly fitting shoes with no support. Corrective insoles or orthotics will help support the feet and protect the ankle and knee joints from misalignment caused by the feet rolling in towards each other.
A shoe fitter will certainly recommend footwear with a firm and supportive sole and with sufficient sole material under the medial arch so as to protect the foot from rolling inwards.