This expression, mostly used in informal contexts, has several different meanings.
When you ‘hunker down’, you sit on your heels; you squat.
Examples: The scouts hunkered down around the campfire and drank hot soup.
The expression can also be used to mean ‘to take shelter’.
Example: The blizzard compelled us to hunker down in a cave for a couple of days.
During a strike, if one of the two parties ‘hunkers down’, it very stubbornly sticks to a position. The party remains adamant and refuses to make any compromise.
As the management hunkered down, unity among the union members began to crumble.