your text book should explain it, but if not think about it like this:
"for all x, if x is a Lion, then it is fierce," which means, "every lion is fierce."
would mean that "everything is a lion and everything is fierce."
And the problem with
is that it means "if there are no lions then coffee is not drunk." So, if the antecedent is false ("there exist no lions") then the conclusion is true. "If some lions exist, then nothing drinks coffee (or coffee isn't drunk)."