What do the oxygen canisters symbolize in Into Thin Air?

The oxygen canisters are symbolic of life. 
Perhaps you have heard somebody use the phrase "breath of life" or "breathe new life."  In Krakauer's book Into Thin Air the oxygen canisters are not a metaphorical breath of life.  Those oxygen canisters are very literally a breath of life for the men and women climbing Mt. Everest.  
High altitudes are dangerous.  Mt. Everest definitely counts as a high altitude mountain.  That is especially true since the last little bit of the climb is through the "Death Zone."  That zone is anything above 8,000 meters.  It's known as the Death Zone because the oxygen amount in the air is so low that a human body is quite literally dying at that altitude from lack of oxygen.  The air at sea level contains roughly 20% oxygen, but at the top of Mt. Everest, the air holds less than 7% oxygen.  
With that little oxygen available, the human body runs all kinds of risks.  High altitude pulmonary edema and high altitude cerebral edema are two of the most severe risks.  Hypoxia is another common risk, and it is the risk that Krakauer and many of the climbers suffered from.  Hypoxia is oxygen starvation to the brain.  One of the early signs and symptoms of hypoxia is severe confusion.  I've attached a video of a hypoxia demonstration.  Watch how quickly confused the person becomes after less than four minutes at 25,000 feet without supplemental oxygen.  In the book, Krakauer wrote over and over again about how concerned he was about his oxygen consumption rate.  He knew that without supplemental oxygen being fed to his system, he would likely die on the mountain.  

By that point, entire sectors of my cerebral cortex seemed to have shut down altogether. Dizzy, fearing that I would black out, I was frantic to reach the South Summit, where my third bottle was waiting.

That's why it was such a huge blow to Krakauer and the other climbers when Andy Harris reported that all of the stashed oxygen containers at the South Summit were empty.  

But Andy Harris, still at the oxygen cache, in-the throes of his hypoxic dementia, overheard these radio calls and broke in to tell Hall-incorrectly, just as he'd told Mike Groom and me-that all the bottles at the South Summit were empty.

Unfortunately, Andy Harris was suffering so severely from hypoxia that he incorrectly read the oxygen gauges on all of those bottles.  They were in fact full.  Had Andy Harris not been suffering from oxygen deprivation, he would have been able to correctly tell the other climbers that oxygen was waiting.  Had that happened, Rob might have been able to successfully bring Hansen down the Hillary Step. But with no oxygen, there didn't seem to be much hope at life.  As I said earlier, the oxygen symbolizes life.  
 

 
http://i.cbc.ca/1.1589291.1379076594!/httpImage/image.jpg_gen/derivatives/original_620/everest-graphic.jpg

https://www.higherpeak.com/altitudechart.html