What is "a canary in a coal mine"? [closed]

I've found that expression on a website, and I don't understand it.

At first I thought it was because the canary is yellow and the coal black... but it doesn't make much sense.

A canary in a coal mine is an advanced warning of some danger. The metaphor originates from the times when miners used to carry caged canaries while at work; if there was any methane or carbon monoxide in the mine, the canary would die before the levels of the gas reached those hazardous to humans.

See here.

Miners used canaries to detect carbon monoxide and other gases in the mines. As long as the canaries continued to sing, the miners were safe. If the canary died, they'd evacuate the miners. I didn't see a reference included in your posting so lacking the context of the quote, I'd go out on a limb to say that what is meant is either literally a reference to that practice of gas detection or it is some sort of metaphor for some sort of early warning system as in the example below.

An example of how this expression can be used metaphorically, from wisegeek.com:

"Today, the practice of using a bird to test the air supply has become part of coal mining lore, but the ideology behind it has become a popular expression. The phrase "living like a canary in a coal mine" often refers to serving as a warning to others. The actual canary had little control over its fate, but it continued to sing anyway. In one sense, living this way indicates a willingness to experience life's dangers without compromise."

In case where a person is described as a canary in a coal mine, it usually means that they are being unwittingly used in some experiment. Like the first person to try to walk across a rickety rope bridge, or across a mine field. This is the same situation as what are called advance parties or scouts. The difference is that scouts are trained, equipped and aware of the risks. The 'canary' has no training, is not equipped, has no choice and may not realise that the task they are attempting is extremely high risk. You may want to compare this to 'cannon fodder'.