Alpha Channel

Extra information stored with an image to define  transparent areas used for keying and superimpositions in other words it determines the ratio of the color that will appear when a pixel falls on another pixel. Also called an alpha mask. The jpeg file format supports the alpha channel, while the png format does not.

Transparency is not available in file formats that do not support the alpha channel.
If the alpha is 0.0, that region will be completely transparent.
If the alpha is 1, that region will be opaque.
If alpha is intermediate, that region is transparent.

For example, you can even decube with the alpha channel. When you save the selection area in Photoshop, it creates an alpha channel in the channel section.