What does it mean that false teachers are trees whose fruit withers?
The analogy means false teachers appear promising and generate excitement, but their fruit withers before it is ready to eat. They never satisfy the inner longing or peace people seek; their apparent goodness proves worthless, leaving empty promises.
Les explains that false teachers are like clouds without water: they get people pumped up so they think they are really getting into something, but it never satisfies that inner longing or inner peace. He says they are like trees whose fruit withereth, and the implication is that the fruit withers before it is ready to eat. It looked so good coming along, and then all of a sudden it is not worth eating. He also describes them as twice dead, plucked up by the roots, raging waves foaming out their own shame, and wandering stars reserved for the blackness of darkness forever.