Supination resistance is a concept based on how much force is needed to supinate the foot.
As a clinical test is it frequently used to help estimate how much force is needed to supinate the foot. This is considered important in foot orthotic therapy in determining how much force may be needed from the foot orthotic to affect foot biomechanics. If the supination resistance force is low then not much force will be needed from a foot orthotic. If the supination force is high, then a lot of force is going to be needed from the foot orthotic to change foot biomechanics. In these cases the rearfoot of the orthotic will need to be more inverted and the material that the foot orthotic is made from needs to be more rigid.
Furthermore, if the supination resistance force is low, the peroneal muscles tend to need to work harder and are at greater risk from injury. If that force is high, then the posterior tibial muscle has to work hard and it is more predisposed to injury because of that.
An episode of PodChatLive did a deep dive into supination resistance: