An Alpha channel is an additional image channel just like the red, green or blue channel. It can contain any information in form of a grayscale image but commonly it is used to store a selection or a mask.
- Black alpha channel pixels usually stand for masked, invisible or unselected pixels in the main image.
- White alpha channel pixels stand for unmasked, visible or selected pixels.
- Gray alpha channel pixels stand for more or less translucent pixels according to the brightness of gray.
Both enblend and smartblend use the alpha channel to determine seam position when blending, see the how to use enblend for patching zenith and nadir images tutorial for details.
More information on the wikipedia alpha channel page