What is the meaning of [Tomorrow is another day]

‘Tomorrow is another day’ is famous for being the last line of Margaret Mitchells’s American Civil War novel Gone With The Wind, 1936:

Scarlett O’Hara: “Tara. Home. I’ll go home, and I’ll think of some way to get him back. After all, tomorrow is another day”

The expression joins “Fiddle-dee-dee!” and “Great balls of fire!” as lines spoken by Scarlett O’Hara that have become commonplace in the language.

The line wasn’t coined by Mitchell however. It is found in print in several sources, for example, Harper’s Weekly, May 1857:

…never losing sight of that, to him, great and glorious fact, that “tomorrow is another day.”