The Apollo's Belt (or iliac furrow) is a colloquial term describing the groove that sits between the thigh and the abdomen.
In the illustration, note the pink dashed line indicates the location of the grooves. The grooves emerge from the hipbone, just above the iliac crest, indicated in green. The green dashed line indicates the inguinal ligament, a band running from the pubic tubercle to the anterior superior iliac spine (ASIS).
Therefore, The Apollo's Belt is a great visual landmark from which we can locate the pelvis.
Learning how to place these landmarks will prove very useful in drawing the human form accurately.