The alpha parameter on a color command refers to the transparency of that color. An alpha value ranges from 0 to 100. A value of 0 means that the pixel is completely transparent. A value of 100 means that the pixel is opaque and will completely overwrite the pixels below it.
When alpha is set to a value between 0 and 100 (55.5 for example) the color will be blended with the color below it.