What does "silver lining" mean?

An element of hope or a redeeming quality in an otherwise bad situation, as in The rally had a disappointing turnout, but the silver lining was that those who came pledged a great deal of money. This metaphoric term is a shortening of Every cloud has a silver lining, in turn derived from John Milton's Comus (1634): “A sable cloud turns forth its silver lining on the night.”