This is a condition in which, for unknown reason, bits of endometrium wind up implanted in the myometrium (see Figure 4 of Diagram: Female Genital System — Anatomy). Sometimes this happens throughout, sometimes just in a few places. As such, the uterus is often enlarged.
The main symptoms are heavy menstrual bleeding that’s often painful. A third of women with Adenomyosis have no symptoms at all. Also, many have other conditions that can cause similar symptoms (fibroids, polyps, etc.), so it can be hard to know which is responsible. There’s no indication that Adenomyosis is related to infertility, and probably has little or no effect on pregnancy as well.
There are no dangers to the condition, except for uncomfortable symptoms (or Anemia if bleeding is heavy enough). Diagnosis can be suspected by ultrasound, or more accurately by MRI, but these tests are only used to identify more serious causes of symptoms. The only treatment that’s completely effective is hysterectomy, though a variety of hormone therapies can be tried for women who still want to have children. See our topic Uterine Fibroids for a brief discussion the risks and benefits of different methods of hysterectomy.