Lildaria Resident started to do a tutorial on Masking in Second Life™. But, she found a good tutorial already exists. She reprinted that tutorial. See: Tutorial: Understanding Mask Mode.
In the tutorial it is talking about masking meaning alpha layers, hiding parts of the avatar or things in Second Life. If you aren’t clear on how this invisible, transparent, and translucent thing works, check it out. I think it will help.
If you are trying to understand how to implement ‘masking’… I think this tutorial falls a bit short. But, that may just be me not getting the ‘how to’ part of it.