Zen is fairly easy to explain, it is also impossible to explain.
This is because the word Zen can refer to two different things.
There are two kinds of Zen. There is Zen which refers to the practice of meditation and there is Zen that refers to the experiences that arise out of Zen practice.
Zen practice is easy to explain. John Daishin Buksbazen does an good job in his book Zen Meditation in Plain English.
But Zen, when it refers to the experiences of Zen practice, are beyond words. Zen will teach you that words are both helpful and a hindrance. They can point at the moon, but never give you the moon.
Remember my earlier post about what Bodhidharma said?
He reportedly taught:
A special teaching outside the sutras. Not depending on words and letters. Directly pointing to the mind. Seeing into one’s true nature and attaining awakening.