The alpha value of a color controls its transparency. Enabling alpha blending with AlphaBlendEnable allows colors, materials, and textures on a surface to be blended with transparency onto another surface.
Alpha blending is determined by the SourceBlend and DestinationBlend properties. With the default BlendFunction, the source pixel times the SourceBlend is added to the destination pixel (the pixel on the back buffer) times the DestinationBlend to produce a color that is a blend of the two pixel colors. The BlendFunction determines the operation used on the outcome of the source blend and destination blend components.