hey s & b,
I’ll answer your question. I was interested in this effect also, but I too found the wording to be a little confusing, and I’m not new to AE. Basically the step you are wondering about is telling you to create a mask using the pen tool and animate the position of the mask over a period of 2 seconds. You’ve probably seen this link, but you can see the animated mask in motion here: https://www.creativecow.net/articles/eriksson_emanuel/sand/index.html
If you are unfamiliar with the pen tool it is pretty simple to use here. Click on the solid you created in the composition window and it will automatically make a new mask for that layer. Continue to click out a wavy edge with the pen tool- as seen in the above link. Click on youu solid in the timeline and hit “M”- this will bring up your mask shape. Set a keyframe at 00 seconds. Move to 02 seconds on the timeline, then move the mask points in your composition window accross the screen, being sure to leave some mask points at the starting point so the white solid covers the majority of the screen.