What are the indents on my butt and how do I get rid of them? (Photo)

You photos suggest you have normal anatomy.  In thin patients there is usually a mild depression laterally where the gluteus muscle is defined at its lateral border near the hip joint.

Hello dear!
Thanks for the pictures and provided information as well.
Based on your photos, you are a good candidate for a small amount of liposuction and fat transfer, to fix your problem and improve your buttocks, and I think you'll get very good results.
l recommend you to make an in-person consultation with a board certified plastic surgeon for a proper answer and to talk about your goals and anything you need to know.
Good luck :)