Apron– If you think of an apron as something that sits on top of a separate article and hangs down to cover it, you’re pretty much right. An apron in the kitchen is worn to protect the clothes you wear underneath. In architecture, the apron doesn’t really protect what lies beneath, but it does sit on top of the under-layer as the fabric apron would.
The window apron serves as a transition between the window sill and the wall it’s embedded in. When you look at the window from the interior side, the flat, broad piece of trim immediately beneath the sill is called the apron. It provides a decorative element. Without the apron, the window would look abrupt and unnatural. It doesn’t protect the wall, but it does hide the seam where the window was inserted.