What is the meaning of ''For of all sad words of tongue or pen The saddest are these 'it might have been' ''?

Out of context: This is the core of regret. Not of regret for past deeds or wrongs done, but for opportunities lost. For most situations in human experience there is a finite window of opportunity. When that window has closed, it is often closed forever. The author is saying simply that there is no greater sense of despair or loss, sadness, than to realize that something could have happened that was not acted upon. This is a line from the poem "Maud Muller" by John Greenleaf Whittier. In the context of the poem: This is still about regret, the road not taken. Young Maud had met a man in her youth who found her attractive, and whom she found equally attractive. She desired for happiness, and to pass on her good fortune by marrying a fair man, and a man of means. By either of them not taking the opportunity when it presented, Maud lived a life of hardship and sorrow that left its mark on her. Her regret was externally visible as well as internally felt.