Two types of blood vessels

Arteries carry blood containing oxygen from the heart ( and lungs ) to all parts of the body. They are under higher pressure and they pulsate.
Veins are the vessels that bring blood back to the heart and lungs. They are under lower pressure, but have to overcome gravity.

How does blood flow against gravity?

Blood in the legs has to overcome gravity to rise to the heart, particularly when standing. Inside veins there are one-way / non-return valves, which open with forward flow to allow blood back to the heart. Backward pressure will close the valves to prevent reversed flow ( reflux ).

Deep and superficial veins of the legs

There are two parallel routes for blood to flow up. Blood in deep veins inside muscles is squeezed forward by muscular activity, drawing in blood from more superficial veins just under the skin. Both sets of veins are connected by perforator veins at points going up the leg. There are two main superficial veins, one draining to the back of the knee, the other reaching the groin and connecting to the deep vein here.