Why do people toast the Boy Who Lived?

In the first volume of this series, we are able to see back to the beginning of Harry Potter's life among muggles (non-magical people). For his safety, he is placed with his muggle relatives by leaders in the magical community at Hogwarts. The book mentions that, at this time, "people meeting in secret all over the country were holding up their glasses and saying in hushed voices: 'To Harry Potter—the boy who lived!'" This is because the infant Harry Potter recently survived an attack by Voldemort, one of the most powerful and sinister wizards in the world at the time. Harry's parents, along with scores of other witches and wizards, were killed by Voldemort. Yet Harry survived and in fact injured Voldemort in some way.
We understand from the story that Voldemort has been accruing followers and power, with dark, murderous intentions. He is a force to be reckoned with and poses a terrible threat to innocent people, magical and muggle alike. The fact that a baby was able to thwart his seemingly unprecedented power gives people hope that Voldemort is conquerable—that there are means that may be used to defeat him. People "all over the country" toast Harry and his survival because it shows there is hope for regaining peace in the midst of Voldemort's campaign of murder and chaos.