When you erase something, you are basically creating a mask. Or in other words, the eraser tool DELETES the portion of the layer you’ve used the eraser tool on. Therefore, whatever is BEHIND it will show up in the spot you’ve erased. This would be another layer or your background layer.
If you don’t want the background to show, you have to either turn off/hide the background, put a layer between the BG and the eraser layer that shows what you WANT to see through the erased hole, or change the background color.
Tiffani B