First, I think that we should rewrite this quote with better
punctuations to make it easier to understand:
"Courage is resistance to fear, mastery of fear - not absence of
fear." (Mark Twain)
I think that "Courage is resistance to fear" means that it is
sort of an antagonist to fear. That's pretty straight
forward...
Then, "mastery of fear - not absence of fear" probably means
that someone with courage will still have fear; it doesn't mean
that he won't have or feel any fear, however he will use his
courage to "master", control, and/or manage the fear and get passed
it.