What is meant by canary in a goldmine?

Answer 1

Something that gives a early warning of greater damage to come. Miners used to--and maybe in some places still do--take a bird underground to detect poison gas. The bird would die first, and if the humans see the bird dead, they know to get out of the mine.

This is sometimes used as a figure of speech for other things, as watching the economy of a small country that is very dependent on a particular resource. If that small country goes bankrupt, it could be a warning to larger economies that are also dependent on the same resource.

Answer 2

A canary was brought down into the mine to detect for dangerous gasses. These gases were things such as methane or carbon monoxide.

Answer 3

Canaries are a breed of bird.