Meaning
Real-world application is the ultimate test of both the tool and the obstacle.
Usage Context
Used to encourage someone to take action or to explain why a seemingly simple task became difficult upon engagement.

A blade may look sharp, and wood may look soft, but the 'Truth' of the wood's density is only revealed at the point of contact. It counsels the seeker to stop theorizing and start engaging. It posits that we only truly 'know' a situation when we actively try to change or 'cut' it. Resistance is the teacher of reality.
Real-world application is the ultimate test of both the tool and the obstacle.
Used to encourage someone to take action or to explain why a seemingly simple task became difficult upon engagement.
It is the sharp edge that knows the true hardness of the wood. Experience is the only teacher of truth. You cannot judge the depth of the resistance until you have made your strike.