What does this mean

[X-Rayz]

I was looking at a psd and when i saw this it made the layer below it sorta invisible, what is it and how do i do it? Sorry if you cant see it, it looks like a layer with an arrow pointing down to the layer below...

http://img84.imageshack.us/img84/8172/untitledku5.png

[.vision]

thats a clipping mask ;)

[X-Rayz]

Cool, thx for the quick reply