A lay-out is a way to preview an advertisement before getting into the actual production.
It's usually an illustrative but temporary depiction of how the printed ad will look when it has been photographed. For the client it's a very handy tool to see what will be. And it's much more cost efficient than to just start producing an ad in real life, just to find out that the basic idea might not work.
For a lot of printed ads multiple lay-outs have been made to see what would work best. Much easier than to have a set build, and have models on the set when you are not sure yet, what you would like to see.
Of course many of these lay-outs are tested with a target group before being produced for print.