"Old Saying Tells Exactly What To Watch For" by Susan Pearl

"That's the canary in the coal mine".

Some lethal fumes are odorless.  These odorless fumes can also be explosive. I have learned that such fumes can be in the coalmines. So in a coal miners setting a canary in a cage would be taken into the mine.  If the bird was healthy the miners knew the air was alright to breathe.  But if the bird showed signs of being overcome that meant the air was dangerous and to get out.  Either way---"the canary was what needed to be watched for safety".

This expression is used to emphasize what needs to be watched in a dangerous setting. That there is a way of knowing in evaluating a situation for danger and safety
Today there are high tech meters that give readings of the quality of air and sound alarm when the air shows signs of being toxic.
A toxic sign is giving warning for immediate action to be taken or there may be loss of life.
"That's the canary in the coalmine " is like saying "That is what we have to watch for safety to avoid deadly consequences.".
Different settings will have different things to watch for conditions to stay safe. If conditions show a sign that toxic levels are immanent than immediate action has to be taken to prevent the loss of life.  Some deadly situations are invisible and we need to a way to be watchful for the protection from potential harm and tragedy.
Smoke detectors and carbon monoxide detectors are warning devices to take immediate action and evacuation.
In dealing with the COVID-19 pandemic there are warning signs when immediate action will need to be taken for the protection and safety of others.  Knowing what to watch is greatly valuable in the prevention of casualties and catastrophes.