Often in today’s society Courage is thought of as bravery, especially (or even exclusively) in the presence of physical danger.  However, Stoics view courage as a much deeper, comprehensive virtue that’s central to one’s ability to live all of the virtues.  Beyond physical bravery, Courage encompasses mental and moral fortitude, the capacity to look all threats in the face while managing fear and emotional response, the ability to deal with ethical dilemmas and societal pressures.  Courage is not just our response to adversity, rather it is the virtue which counters adversity and balances the scale so that we might live in peace, unaffected by the inevitable suffering of life. 
This virtue manifests itself in the physical world as endurance, confidence, stout-heartedness, perseverance, magnanimity, and hard work.  Courage is the antidote to cowardice, inactiveness, fear, and doubt. 

As Marcus Aurelius advises in Meditations 

“Be like the cliff against which the waves continually break; but it stands firm and tames the fury of the water around it”.  

Stoics place a supreme importance on the ideal of remaining steadfast and resilient in the face of adversity, not giving into emotions, not crumbling against the onslaught of the crashing waves and disappearing into the sea. 

Confidence is not arrogance, but a deep-seated belief and trust in one’s ability to handle the inevitable challenges of life.  The great Roman Stoic philosopher Seneca stated
“Difficulties strengthen the mind, as labor does the body”. 
Much in the same way that your body will weaken and wear down if physical resistance is avoided, so too will your mental fortitude - your strength of mind and moral conviction - deteriorate if you hide yourself away from the challenges that face you.