Anti-aliasing is a technique which reduces this by coloring neighboring pixels. This process though, is very slow and I advice you only to use it, if you can live with the poor performance. Later, when we see texture mapping, we will see other techniques to reduce texture aliasing. If Alpha equals 255, the color is shiny and bright. This technique is frequently used to simulate lights.
Alpha values can come from other sources as well.
But for know, we'll stick with constant and iterated Alphas. End of introduction. The only thing you should remember is that Alpha isn't a real color. We use Alpha to transform the RGB portion. You should also understand the AlphaCombine function.