Glue was a type of surface that could hold together two parts. The apply button was located in a toolbar near the top of Roblox Studio. By using it, a user could have held multiple bricks, parts, constraints, seats, solid models and more.
When a user selected the Glue button the icon would appear with a hand with crosshairs pointing to a yellow brick. When they selected the surface to apply it to, the Glue texture would appear. It could have also be applied to Surfaces in the Properties window of the object, with labeled surfaces. Glue doesn't hold together, say for instance a destructive object interferes with the texture. This will tear the bricks apart, leaving the side with Glue exposed.
In 2015/2016, Glue was removed and was replaced with Welds. When a user tries to add the Glue texture in Roblox Studio, it turns into the Weld texture.