This is a concept based on how much force is needed to supinate the foot. The concept is that different feet need different forces to supinate them. This means that different structures will be overloaded if that force is high or low. It also means that different amounts of force are needed from foot orthotics if that force is high or low if the foot orthotic is going to change the position or alignment of the foot or if it changes it too much.
By way of example, if the force to supinate the foot is low, then the peroneal muscles have to work harder, predisposing the tendons to peroneal tendinopathy. If the force supinate the foot is high, then the posterior tibial muscle has to work harder, which might increase the risk for posterior tibial tendonitis. If that force is high, then a more rigid inverted foot orthotic that supplies more force is going to be needed to overcome that force.
This video from PodChatLive on the supination resistance test discussed with all the key researchers on the topic:
There have also been plenty of discussions on supination resistance on Podiatry Arena. The most comprehensive article online is this one: The concept of ‘Supination Resistance’ and this encyclopedic entry on supination resistance.