Banding is when you have two lines hugging each other.
Be careful, as banding can also occur diagonally.
Methods of fixing these banding issues.
Banding can make the lines look fat, and make specific pixels stand out. All in all it's not a good thing to do.
I've highlighted all the lines that are banded in your hat with either a red or yellow highlight.
Instead of trying to fix the shading in the white, it should be removed all together, as it's not being shaded to reflect the shape of the hat, but is instead being shading randomly based on the colors in the area. Also known as pillow shading.