How do I Understand Masks and Alpha?

Alpha

Alpha relates to whether something is transparent or opaque. Transparent being how well we can see through it. How clear is it. If something is clear, light easily passes through it. If it isn't clear, then it is opaque and can't be seen through. If an object is 100% opaque, light doesn't pass through it. If it is fully transparent, light fully passes through it.  This is why you see a background grid when you set a background to transparent in photoshop or blender. The background is technically rendered as fully transparent, see through. Therefore, we can't see the materials or color of the background. 

So the key is to remember alpha how transparent or opaque is it,  how much light can pass through. 

Alpha channel is simply the information that relates to how transparent or opaque that image/video/tetxture/material is. It will not always be present/needed in an image or video.  

Masks

Think of how a mask you wear on your face covers your face vs what someone looking at you can see. An eye mask will block off the areas around your eyes and over the bridge of your nose from sight. Your lips, some of your forehead and lower cheeks are still visible. 

Now think of a stencil. The stencil holes allow you to paint the object it is attached to, while the solid areas masks off the object portions that will not be painted. 

Think of applying a mask to an image as covering an area you do not want to be touched by a change we want to make to the image/video. We can also use a mask to hide things.

A common usage is if we want to change the background color. We apply a mask to the main image, then when we apply our new color and it will only change the background. 

Black is applied as your mask. White are the areas you can change.  Black is a value of 0 and white is a value of 1. On or off. 

When you get to more advanced masking, you can use a gradient. But you will come to that.

I think you were wondering how alpha and masks relate to each other? You can set the masked off areas to be opaque and the non-masked to be translucent, or vice versa. If you applied a gradient mask, then you could have gradient translucency/opacity. 

I hope that is helpful as an explanation of alpha and masks. I am sure the instructors will have classes/tutorials they can point to, if not stronger explanations. I honestly think that they could do an entire course focused on masking in Blender.