What is the meaning of every cloud has a silver lining?

All 'bad' things come with some 'good' attached to them. Out of every problem we face we can gain some advantage.
The expression comes from the fact that many dark clouds are lighter at the edges, appearing to be white, or silver.

The usage came from the American Civil war. While today this phrase is one of hope and optimism-i.e. though the skies are dark, something good is still there. Originally though, this phrase would have meant just the opposite. The "cloud" referred to was the plumes of smoke from the enemy artillery, and the "silver lining" was the glint of morning sun off the artillery in the background. Since many of the battles would start just before dawn, the soldiers were sometimes facing a confusing look at fog and smoke, in the predawn light. The veteran soldiers would tell the newcomers-"avoid the clouds with the silver lining", and soon those rookies would learn that "not every cloud has a silver lining" meaning they could head for the fog, which was doubly better as the haze was not from the artillery, and the fog itself offered some concealment. When the soldiers came home from the war, they would sometimes use the phrase "not every cloud has a silver lining" to mean that there were sometimes unexpected good things; but through the course of usage by those not knowledgeable of its origins, the "not" was soon dropped from the phrase.