This is an anatomical area anterior to the rectum, which can be felt on digital rectal examination. In males, it is the rectovesical pouch (between the rectum and the bladder) and in females, the rectouterine pouch (between the rectum and the uterus).
Part of the reason it is of interest to doctors and anatomists because it is the lowermost portion of the abdominal cavity when the subject is standing up.
Occasionally, a hard mass which is a metastasis from a primary carcinoma somewhere in the peritoneal cavity may present there - this is called Blumer's shelf and is palpable on a digital rectal examination.