A definition first: The endometrium is the mucous membrane that lines the uterus. For women who have had hysterectomies, the endometrium is not an issue in planning hormone therapy (HT).
For others, the endometrium is a "target tissue" (like many others) for estrogen and progesterone. During our reproductive years, those hormones signaled the lining of the uterus to thicken (proliferative endometrium influenced by estrogen) and then to shed (secretory endometrium influenced by progesterone), over and over in our menstrual cycle.
Endometrial cancer is a well-recognized consequence of "unopposed estrogen," a continual message to proliferate and thicken without the proper "opposing" influence of progesterone. Nearly all endometrial cancers will be "estrogen influenced."
When we plan HT for a woman in menopause with a uterus, we must balance estrogen and progesterone. (And, in fact, for a woman in reproductive years who doesn't ovulate, which typically triggers progesterone, we'll compensate with progesterone therapy.)
As with most cancers, there are factors we can't always explain. Obesity, however, is the most common risk factor; in fact, obese women are at higher risk than their friends on HT including both estrogen and progesterone. Fat (adipose) tissue produces estrone, an estrogen that is very weak but does influence the endometrium. Sometimes we biopsy obese women and find "precancer" of the endometrium; part of our treatment is progesterone in an effort to reduce their cancer risk.
Just one more reason, I'm afraid, to make healthy habits a priority—and to work with your health care provider for HT that takes your health history and priorities into account.