Joshu went to a hermit's and asked: "What's up? What's up?"
The hermit lifted up his fist.
Joshu said "The water is too shallow to anchor here.", and went away.
Joshu visited the hermit once again a few days later and said: "What's up? What's up?"
The hermit raised his fist again.
Then Joshu said "Well given, well taken, well killed, well saved."
and he bowed to the hermit.
"What's up? What's up?" menas literally "Have you any Zen?".
Provided that this koan hasn't been mistranslated, it seems to deal with interpretation.
Joshu gives the hermit such a broad question that literally anything would answer it.
We don't know why the hermit wishes to respond by lifting a fist. Maybe he does to in angry rejection, maybe he does so to in acceptance to calmly show zen.
Joshu declares that he won't interpret the hermits gesture as anything, and goes away.
Later Joshu comes back. If the hermit now responds to the same question in another manner, then he may draw conclusions whether or not that gesture will be angry or calm too.
However, the hermit responds in the exact same way. Does the hermit just have a narrow repertoire of responses, or is he doing this purpose, just to mess with Joshu's ability to interpret his intention? We don't know.
Joshu now has to draw the conclusion that he can't draw a conclusion about the hermits intention, which he states by responding to all the intention alternatives.
Joshu then bows to the hermit, because whether or not the hermit meant to teach him this, the lesson does in both cases make the hermit deserving of a bow.
This koan is about how things have more meanings than just one. Often they can have opposing meanings. When you become enlightened enough that you see through dualistic concepts such as good or evil actions, a lot of things lose their inherent meaning, leaving you is a fuzzy world where any hasty interpretation can be right or wrong. In the case of this koan, Joshu is cornered by his own wisdom.