"if you can force your heart and nerve and sinew
to serve your turn long after they are "
-'if' by rudyard kipling. what did he mean by that?

This poem means that if you just continue to put an untiring effort with a sharp will even your heart and nerves are worn out and hold on with what you are doing or what your goal is. 
You will achieve success and greatness. The fruits of your labors will serve you even when you are long gone in this world.

Not sure this is a hard one huh