The Reinforced Glass is a mostly transparent glass-building-block with an iron frame on two sides, as well as with iron mullion and transom on four sides.
It can be crafted in your Crafting Menu (default key "Q") or at Crafting Tables (that still exist in worlds created before R22 on September 16th 2015), but only after the crafting-recipe has been unlocked by
- forging Glass by putting blocks of Sand into a Forge
- creating Iron Rods by putting Iron Bars into a Processor. Iron Bars can be smelted from Iron Ore extracted from Nodes found on the Stalactite layer or collected from Iron Treasure Chests and Diamond Treasure Chests
- crafting Siltstone Walls (unlocked after mining Siltstone with at least an Obsidian Mining Cell)
- crafting Iron Walls
To craft 8 (blocks of) Reinforced Glass at a time, you'll need (as of R44 in June 2017):
The block can be rotated by pressing and holding R and moving the mouse while holding the left mouse button.