Alpha channel

an image channel containing a mask or selection. 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 wikipedia:alpha channel[*] page