What is the significance of "defamiliarization" in William Carlos William's poetry, and how he has used this technique?

Defamiliarization is the poetic process of taking ordinary objects and using them in a poem in such a way that the reader sees them as new or different. Williams often used common things in his poems, but in his poetry they become unusual, or have a greater significance than they otherwise might.

Take, for example, "This Is Just To Say":

I have eaten
the plums
that were in
the icebox

and which
you were probably
saving
for breakfast

Forgive me
they were delicious
so sweet
and so cold

The content of the poem is a simple note apologizing for eating someone else's plums, but the method of its presentation foregrounds the note and makes it seem more important than you might ordinarily think. Here this is done by the form of the poem, especially in the line breaks. These breaks serve to cut the note up into little pieces which demand to be considered separately, and in isolation from, the other lines.

Consider the different effect if we remove the line breaks, and add some punctuation:

I have eaten the plums in the icebox, and which you were probably saving for breakfast. Forgive me! They were delicious -- so sweet and so cold.

Now we have a simple note, in a friendly and apologetic tone -- but not very memorable. Put the first two line breaks back in and we have to consider each bit on its own:

I have eaten
the plums

These two lines can function together, but they also function separately, in that we have to first consider the (perhaps ominous) act of having eaten, and then, quite separately, the plums, which, while using totally neutral language, nevertheless brings before us an image of the fruit. You can seen the same effect at work in line seven, where the single word "saving" gets its own line. We are forced to consider the word outside the context of the note: What is being saved here, or who? 

In this way, Williams' poetry forces us to understand the importance of everyday objects, and encourages us to pay attention to everything around us.

Source: Altieri, Charles. “Presence and Reference in a Literary Text: The Example of Williams' "this Is Just to Say"”. Critical Inquiry 5.3 (1979): 489–510.