Typically for keeping your brain safe and maintaining the physical structure of your head.

You can interpret it in many ways. The most basic thing would be that it symbolizes mortality.

Further we see a cross, a whip, books, and of course the flame and the smoke. Her naked shoulders being naked is interesting too, as it would’ve been considered an intimate part of the body.

From my point of view she is in deep contemplation (books, expression) about life (fire) and her mortality (skull, smoke), her sins (naked shoulders, smoke) and her transformation to a spiritual life (fire).

There’s more nuance to it but it’s just to give you an idea how to read it, everybody has their own unique experience and interpretation, and there is no right way to do it.