It is the fold that lies just under the buttocks and it is indeed banana shaped.
It is created by the weight of the buttocks which pushes the skin and fat of the upper part of the thighs into a bulge.
The treatment lies both in conservatively removing fat at the bulge itself and eventually transplant it both in the crease of the buttocks and under the banana fold.
Sometimes the fold is so large, it needs to be removed surgically. This is called a Buttocks lift .The fold is stripped of it's skin and buried under the buttocks. this obviously creates a longer scar, which however can be very acceptable.