Cornices are overhangs of snow susceptible to breaking. While hiking, one should stay away from them, both on top and below. They may not be easy to spot when you are on one. Here are some resources from the Net.

http://www.fsavalanche.org/cornice/

https://www.genuineguidegear.com/life/blog/g3/bombs-backcountry-how-be-safe-cutting-cornices

And you might have heard about  this recent tragedy too.

And of course you may like to check out avalanche.ca for forecasts before heading out.