The adhesive is an alien substance that can patch almost anything.
In "Dipper and Mabel vs. the Future," Ford and Dipper venture into Crash Site Omega to find some alien adhesive in order to patch the Interdimensional rift and prevent it from breaking. Ford warns Dipper to be careful when handling it, as it could "seal up all the orifices in [his] face".
During the search, Dipper's Magnet Gun gets stuck to an hexagonal object which contains the adhesive.
Later, Ford has the adhesive ready and asks Dipper to hand him the Rift so they can finally patch it up, only to find out that Mabel Pines had uknowingly run off with Dipper's backpack, taking the rift inside.
The adhesive is a slimy, bright purple liquid that acts as a powerful glue.