What do you call the short barrier outside window?

"window ledge" would be a small, slanted, shelf-like bottom to the outside of the window.

I think that would still be called a ledge.  too small to be a balcony.

a ledge

It does not look like a "barrier" ; it is not "barring" access to anything.  There is not enough information.

What is its relation to the window ?  Is it at floor level (with the apartment), or is it raised to the level of the window ?  Is there access to it from inside the apartment (does the window open to allow access ?).  Does it have a meaningful function (such as to allow the outside of the window to be cleaned ?).

As a stop-gap reference name for it, I would call it simply a "platform".