To understand how varicose veins develop, one must understand how normal veins work.
Arteries supply blood from the heart to the extremities (hands and feet), delivering oxygen to the deep tissues. Veins act to transport deoxygenated blood back to the heart. Walking leads to contraction of the calf muscles which in turn pumps blood within the veins in an upward direction towards the heart.
What this means is that venous blood needs to move against gravity when we stand and walk.
Due to gravity, blood tries to flow back down towards the feet, in the opposite direction. However, normal veins have valves within them that are very similar to one way valves in gas or water pipes. These valves allow the passage of venous blood in an upward direction (antegrade flow) and subsequently close to prevent abnormal flow towards the feet (retrograde flow).