When using the brush, sometimes you want to remove pixels that you have painted from your current selection. To do so, just press "E" (or click the icon in the toolbar) to switch to the eraser. As the brush and the eraser are complimentary tools, the selection you have made with the brush will not be removed. Instead, you will be able to erase parts of your current selection in the same way you would add to the selection when using the brush. When you have finished erasing, you can switch back to the brush by pressing "B".