Need to fire someone? That's probably going to be a crucial conversation. Need to talk to your spouse about the fact that they are having an affair? That's almost certain to become to be a crucial conversation. Need to gently, but firmly ask your pair-programming coworker to do something about their chronic halitosis? Even that could turn into a crucial conversation.
And, also according to the authors, how we handle these conversations has a measurable impact on our success in life. If we withdraw from these conversations (or "go to silence" in the book's vernacular), we risk not ever getting our needs met. If we force the issues (or "go to violence"), we risk our long term relationships with those involved.
The best among us handle these conversations honestly, earnestly, and without hurting others. They express their needs, listen to the needs of others, and work toward a mutual purpose while maintaining mutual respect.
To be honest, I found some of the imagery in this book a bit silly. ("Shared pool of knowledge?") But, I found the underlying message about how to talk about difficult subjects with literally anyone extremely useful, both professionally and personally. It has provided me with much fodder for thought. Ultimately, I think it would be most useful in an environment where an entire team reads the book. That way, the silly vocabulary will be a shared point of reference the next time tempers flare.