What does it mean?
God, grant me the serenity
what is serenity? Serenity is defined as being calm, peaceful and untroubled. When we are in a serene state we can accept things more readily. Asking for peace and to be untroubled is what this line is referencing.
to accept the things I cannot change,
we need to understand that we cannot change everything. It is not possible to change all things and when one can accept this they can move on to what they can change.
the courage to change the things I can,
change is difficult and frightening. It is the fear of the unknown that leads us to question ourselves. It takes courage to make the necessary changes in your life. These changes will not all be easy, but they will be worth it in the end.
and the wisdom to know the difference.
sometimes it is difficult to understand the difference between what we can change and what we cannot. Feelings can get in the way and we can end up trying to change things or people we cannot change. Change starts from within and works its way out. Knowing the difference is the hard part.