Economic Nuclear Winter

I have been hearing a lot that banks have a worse case scenario strategy to prepare for an economic nuclear winter that could happen at the end of this year.

I am curious on:

What exactly is a economic nuclear winter? How will this arise, and what are the major factors that would cause this? How would this affect the overall economy, and more specifically the American economy? What type of strategy are banks using to hedge against this risk? ( i believe most bonds are still fucked so what are the safe options) Any ideas for when hedging your personal equity investments?