What does the word "empty valley" mean and what are its metaphors?

My understanding should be shouting in the empty valley, and there should be many echoes in the valley.

Metaphor has a great response and is an unprecedented move, which has triggered a group reaction of newcomers.

It has something in common with the sentence "shake your arm and shout, and those who respond will gather", but the sentence "shake it" does not mean that it is the first one.