This building block is a slope corner with two sides cut in half and one recessed side.
The slope corner has a thin iron frame all around and slightly murky glass inside.
Reinforced Glass Slope Inner Corners were implemented with update R46 (Galactic) in September 2017.
Cutting blocks in a Processor does not require any crafting recipe.
The slope corner blocks can be rotated by pressing and holding R (as the default key) and moving the mouse while holding the left mouse button. The chosen rotation angle can be "locked" too by simply typing "r" while looking at a rotated block or item, so next all items of the same stack will be placed facing the same direction.