What is this thing called?...

[ututman4]

http://imdb.com/media/rm3809712128/nm0718646

The thing in that person's hands; what is that item called and what exactly is it used for?

 

[houseofwolves]

Don't really know the name of it, but if I were to guess what it does is, gives the director a look at what lens he should use for DOF purposes and just angles he may want to look at before shooting film.

 

[JZASMM]

It's called a viewfinder.

 

[TC Collins]

It's an adapter that gives the director a view at what the camera will see from a certain angle without actually having to look at it through the camera viewfinder.

 

[RichSB]

It's called a Director's Finder or a Director's Viewfinder.
[Wikipedia article]

The American Cinematographer's Society sells them on their website, or I'm sure you can find them elsewhere as well.
[ASC Store]