What does the quote in the time of darkest defeat victory may be near us mean?

It means that bad times are temporary. Take 9/11 for example. This was a dark time for the U.S. we felt vulnerable and defeated in a sense. Ever since that catastrophe, the security in the airports has really buckled down in an effort to prevent this from happening again. Because it hasn't, it is valid to say that these efforts have been successful.