Glue is a type of surface that can hold together two parts. The apply button is located in a toolbar near the top of Roblox Studio. By using it, you can hold multiple bricks, parts, constraints, seats, solid models and more.
When you select the Glue button the icon that appears is a hand with crosshairs pointing to a yellow brick. When you select the surface to apply it to, the Glue texture appears. It can also be applied to Surfaces in the Properties window of the object, with labeled surfaces
Glue doesn't hold together, say for instance a rocket or another destructive object interferes with it. This will tear the bricks apart, leaving the side with Glue exposed, although it will not attach to another surface.
In the New Materials Update, Glue was removed and was replaced with welds. When you try to add the Glue texture in ROBLOX Studio and in Player Build Mode, it turns into Weld texture.