The metaphor light at the end of the tunnel generally refers to the first glimmer of hope or the first sign of rescue from a desperate or hopeless set of circumstances. People trapped inside a real underground tunnel or cave often become discouraged after hours of walking in complete darkness and uncertainty. Spotting a light would be a sign their struggles would soon be over and escape would now be possible.
Metaphorically speaking, a light at the end of the tunnel would signal the end of uncertainty or doubt. A sudden upswing in the stock market, for example, could be characterized using this metaphor as a hopeful sign to investors struggling through an economy in recession. A decision to withdraw troops from a war zone could also be seen as a light at the end of the tunnel for soldiers' families concerned about their safety. It is usually considered the first sign of a positive change in a course or outcome.
It is important to note that the first light at the end of the tunnel is not necessarily a guarantee of a more positive outcome. There still could be a number of obstacles to overcome or crucial decisions to make before the source of the light is reached. As a more cynical expression suggests, the light may turn out to be a train. The significance of the first sign of light is a much-needed suggestion of hope, which often motivates people to keep moving with a renewed sense of purpose.
Tunnels are often seen as man made structures, carved out of mountainsides or underground passages for a higher purpose. The supposition is that most tunnels do have an entrance and an exit, but following the path towards those exits can be a long and frustrating experience. After hours of walking through a dark, cold tunnel, it is easy to see why the first glimmer of light would be considered inspirational. The metaphor of finding hope at the end of a long, difficult journey is indeed comparable to glimpsing the light at the end of a real tunnel.