6. The Tools in the Toolbox / 6.3 Tools for Image Layers
The eraser is used to erase parts of an image. It uses the background color.
The current brush defines the shape of the eraser.
Clicking on Apply To Entire Image clears the whole image (currently selected). Double clicking the tool icon has the same effect.
A quick way to set the background color is holding Shift+Alt and clicking into the image.