In Flash, the technique know as “masking” should really be named “revealing”. It can feel backwards, so just think…REVEAL and it’ll be easier. Here are examples of static masking (like a window) with moving content (something moving past the window), and animated masking (the spotlight) with static content (the text).

Basic Masking :: Part 1 of 2

Basic Masking :: Part 2 of 2

Let me know if you need clarification on any of this.
:o)