Transparent shapes allow light to pass through so that you are able to see what lies beyond them.

Richard Estes paintings are a perfectly judged balance between the transparent, reflective and opaque forms of the metropolitan environment of New York. In his painting of 'Donohoe's', the image is essentially a view into a diner through one of its windows. However, plate glass windows are not only transparent, they also have reflective properties. Consequently, the view of the window becomes a composite of the inside of the diner and a reflection of the street outside.

https://www.artyfactory.com/art_appreciation/visual-elements/shape.html