It’s generally assumed that each foot just has one arch that runs along the inside of the foot. Actually, this is one of three arches.
Thinking of the base of the foot as a triangle, we can draw a line from the heel to the base knuckle of the big toe and then to the base knuckle of the little toe and back to the heel.
There is an arch between each line of the triangle. The biggest arch is the one we know about, along the inside of the foot. The other two are less obvious, partly due to muscular padding.
Combining all three arches is known as the “plantar vault”. Plantar meaning “bottom of the foot”.
When standing, I find it useful to think of this vault giving an upthrust to the foot. A sense of springiness.