P'u (pronounced POO) is literally the "uncarved wood" or "uncarved block."  The metaphor suggests we are all born with our personality like an uncarved block of wood.  All that we experience and all we are taught starts to carve away pieces of that original simplicity.  Taoists try to regain that early sense of unlimited possibility by trying to "unlearn" things until everything becomes a new experience.

We may feel that we must hold fast to a sense of history so that we don't "repeat the mistakes" of our ancestors or at least our own mistakes.  We have an alternative though.  We can use our instincts to make a correct choice in each situation.  And we can do it without an unnecessary burden of past experiences which may or may not be applicable to the new one.

Lao Tzu (Laozi) in the Tao Te Ching (Dao de jing) writes about p'u in chapter 28 as returning to the infant:
TAO
MANOR
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"To know the white,
But to abide by the black,
Is to be the model of the world.
Being the model of the world,
One's everlasting power (te) being full,
One again returns to the uncarved wood."

-- from chapter 28, E. Chen (tr.)
The uncarved block is a symbol of simplicity.  Taoists eschew complications, but that doesn't mean we reject technology.  If there is a more efficient way to do things, that too can be simplicity.

Check out The Tao of Pooh.  Benjamin Hoff uses Winnie the Pooh as an example of Taoist simplicity.
P'u
Uncarved
Block