What exactly is endometrial receptivity?

What exactly is endometrial receptivity?
Endometrial receptivity is a complex process that provides the embryo with the opportunity to attach, invade, and develop, culminating in a new individual and continuation of the species. The window of implantation extends 3–6 days within the secretory phase in most normal women. In certain inflammatory or anatomic conditions, this window is narrowed or shifted to preclude normal implantation, leading to infertility or pregnancy loss. Of the factors that prevent normal implantation and pregnancy, embryo and endometrial quality share responsibility.
Source: fertstert.org