I love this poem! You have to look at the section to answer this question If you can force your heart and nerve and sinew :To serve your turn long after they are gone,: And so hold on when there is nothing in you Except the Will which says to them: ‘Hold on!’ "They" is referring to the heart and sinew (muscle.) This could mean your whole body. "To serve your turn" means forcing your body to keep going even when it's tired/exhausted. So the stanza is saying: if you can continue on living even when your body is exhausted, and hold on (to life) when you have no other reason than the will to hold on. Of course the "then" part comes at the end of the poem.