The Mask feature lets you hide sculpting on a layer by letting you paint opacity values on the model in the areas you don’t want to appear on the final model. The sculpting on other layers is unaffected.
Painting a mask in this fashion lets you non-destructively make edits to the sculpting on a layer that can easily be reversed if necessary.
The sculpting in the masked regions is removed or reduced depending on the properties that you set for the Mask tool.