That’s the bowl shape the underside of the foot (naturally!) takes when healthy (most of the time).

▪️ This 1st comparison shows you “best” vs. “worst”. The left pic has concavity + sole depth. Right pic has neither.

▪️ Here, I highlighted what I see in terms of concavity. Top pic is still the best, middle is still the worst. Bottom is a foot with decent concavity but little solb kidxe depth (that’s to be expected from most horses, coming out of a wet winter: very little stimulation).

A common practice is to “shape” the concavity during trimming, which is carving sole to shape the bowl under the coffin bone.
It is not something I personally do, as I find it detrimental. By taking sole out with the hoof knife, you’re kind of taking off built up “horn” or protection the horse produces to feel comfy.

With time and stimulation, it will gradually form at the horse’s own rhythm!