What does it mean to be stubborn and when is a good time to be stubborn?

Stubborn means to have determination to not change one's attitude or position on something. A good time to be stubborn is when something compromises your morals. 

To be stubborn, showing or even having determination not to change ones attitude or position on something. When you ask "when's a good time to be stubborn" my personal opinion would be when you're standing up for something you believe in and your standing firm on your believe despite what others might say or think to move you. Hope this helps in some way :)