John is warning them of imminent destruction--The ax is lying at the root of the trees; every tree that doesn't bear good fruit is going to be cut down.
The crowd asks him how they can avoid being destroyed.
He tells them "Share what you have with who needs it."
Christians today seem to be upset about behavior of others, but I don't hear much criticism of people neglecting to share. Are we raising a different crop of fruit than what John was talking about?