What is "yetzirah"

Yetzirah (also known as Olam Yetsirah, עולם יצירה in Hebrew) is the third of four worlds in the Kabbalistic Tree of Life, following Atziluth and Briah and preceding Assiah. It is known as the "World of Formation".

"Yetsirah" as in "making" is as opposed to "Beriah" as in "Creating": actually taking whatever matter that was created in "Beriah" and shaping it into the basic elements.