Foreground Alpha is a rendering mode that sets the Alpha (Opacity) pixel depending on whether the pixel is considered to be in the foreground or background of the scene (for example to create a 3D configurator).
If you enable Foreground Alpha, when you save your image in PNG, EXR, or any other format that supports an alpha channel, transparency information will be included in the saved image. This allows you to easily assemble the objects in your scene using a background image.
Here you can see a rendering with activated foreground alpha. The grey checkerboard pattern shows alpha zero pixels completely transparent.