Something that helped me when coming to terms with Alpha Channels was the realization that the values in the image are being multiplied against the alpha.
So if you look at the image provided, we can see that when the yellow pixel with a value of "6" is multiplied against the same pixel in the alpha (star), which is a "1", the result is that the value stays a "6" or yellow. When the blue pixels (23) are multiplied against the same area in the alpha channel (0), the result is 0.
1 = Opaque
0 = Transparent
Keep in mind I'm simplifying this immensely for purposes of clarity. Every pixel actually has at least 3 values corresponding to their levels of Red, Green, and Blue.
I think the word "channel" also confuses people, so just to be clear, an Alpha can be any other image, or it can be a "channel" in the same image. What's a channel? Remember how I said every image has 3 values (RGB)? Each one of those is a "channel". There is a red channel (1), a blue channel (2), and a green channel (3).
There can also be a 4th channel, which is the alpha channel.
Image types that support over 3 channels = png, tif, exr, and others.
Image types that do not support over 3 channels = jpg, gif, and others.
So, you can have an alpha already "build into" your image (channel), or you can have a separate image you use as the alpha. Any image can be used as an alpha, because "using" it, is just multiplication.
Also, keep in mind that there are a lot of values between 0 and 1, which are grey. So if the white star had been grey, let's say with a value of 0.5, the resulting image would be semi-transparent.
When you make a shape in Moho, every pixel that makes up that shape has an alpha value of 1. Everything outside that shape has an alpha value of 0.
Anyway, hope that adds further clarification to this thread!
