It is supposed to be the crest of her pelvis, but its ... very, very unlikely that an actual human being would have a pelvis/toso area that looks like that.

She has insane muscular definition over the pelvis in areas that... there aren't muscles there, that are shaped like that, in regular human anatomy.

Areas of the pelvic/torso region that a regular human has ligaments in, ie, tissue that tethers a human muscle to a bone, are drawn as if they are entirely muscles that could be worked out and grown in volume... (see the image in my above post. yellow is common areas that store fat, red is actual muscles, white is ligaments)

... the drawn figure's pelvic crest is also significantly higher than her belly button, which... is either wildly unrealistic, or wildly uncommon in actual humans. The proportions are off.

...as well as all the other 'not technically literally impossible but extremely unlikely' kind of combinations of anatomical features that make the whole figure in totality very strange, as I already noted.

Basically, whoever drew this has or is appealing to a muscle mommy kink.