I have found this phrase on New York Times recently. And I couldn't figure out the meaning of let freedom ring. I tried searching the meaning on the dictionary too.
Though I may not be here with you, I urge you to answer the highest calling of your heart and stand up for what you truly believe. In my life I have done all I can to demonstrate that the way of peace, the way of love and nonviolence is the more excellent way. Now it is your turn to let freedom ring.