This is referring to the little face in the top right of the UI, or in card previews, etc. that represents your account.
Currently the only thing you can do to customize it is set the color. It would be nice if you could do more things with it, such as changing the face, or uploading an image, etc.
e.g.: A light Mii-like editor could be fun.
I know this is a feature that could be a lot of effort, so Iām not expecting anything anytime soon 