What does the man in the red sweater teach Buck and how does the lesson apply to humans today?

What Buck learns from the man in the red sweater is that
the man with the club has to be obeyed.  In other words, you have to do what the people
with the power want you to do.

You can say that this
applies to people today as it always has.  Might, in some ways, makes right.  We may not
like it and it may not seem civilized, but at times we simply have to give in to the
people with more power.  If we are like Buck we can wait and get stronger ourselves and
then be the ones with the power.  But until we do, we have to
obey.

This is partly an admission that we are weaker than
someone.  But Buck is also smarter than some dogs that fight to the death.  Part of what
he is saying is that we have to be smart enough to fight when we have a chance to win
and to let it go when we don't.