Alpha is a fourth component of colors, in addition to red, green, and blue. It is used in drawing semi-transparent objects, and is thought of as representing the opacity of an object:
For example, if a surface has an alpha of 0.25 (25% opaque), then we would typically expect 75% of the light (i.e. the color) from anything behind it to pass through the surface.
By default, alpha is ignored. There are two main functions that control the use of alpha: