When you have a turkey breast side up it means that you would have the wings of the turkey up and the legs of the turkey down.
So when you place the wings of the turkey up and the legs of the turkey down then you have the breast side up on the turkey.
The breast side down of the turkey is when you reverse the turkey so that the legs of the turkey are up and the wings of the turkey are up.
You can cook the turkey either way as it really does not matter but most people cook or bake the turkey or roast the turkey breast side up.