It's hard to say. The image — one wing on each side of her ankle — could have a number of meanings. They're around her Achilles' tendon, and the wings look a bit like the ones created by Icarus, introducing a few possible Greek mythology references. Or maybe she's just symbolically spreading her wings and getting ready to take off.
Hadid's are some of the most creatively placed tattoos we've seen. Whatever their exact significance, the wings are an empowering image.