The color used for Line, Fill and Font (text) can be expressed in a variety of ways. RGB (Red, Green, Blue) is commonly used where 3 numbers ranging from 0-255 can describe over 16.5 million colors. e.g. 0,0,0 is Black and 255,255,255 is White.
The ARGB model adds a fourth number representing Opacity, or the Alpha-channel, hence "A". A value of 0 means fully transparent, and increasing values increase opacity up to a value of 255 which is fully opaque.
Using the ARGB model you can type in values for Line, Fill or Font colors to specify a level of transparency so, for example, a Fill Color of 0,0,0,0 will give a transparent fill, while 128,0,0,0 gives a black fill with 50% transparency.