mount hides underlying files

[Jay O'Brien]

If I mkdir /test and then place files in /test, those files 
are no longer visible when I use /test as a mount point. The 
files become visible again when I unmount the device.

I have read documentation explaining this phenomenon, and I 
would like to review that documentation again. Is it in the 
handbook? 

A reference or link would be appreciated.

Jay O'Brien