A binding link is a symbol which keeps a demons or any other possessing entity trapped inside its meatsuit i.e. the human it is possessing. The demon can neither escape voluntarily or be exorcised. A binding link may be related to the "binding magic" used on curse boxes to keep the evil inside.
When Meg possesses Sam she brands a binding link into his forearm. When Bobby tries to exorcise her, it doesn't work. Bobby realizes the brand is keeping the demon trapped inside Sam, and burns across the mark with a hot poker, breaking the spell.
- Crowley: This bit of carving will tie our friend here down. No zapping off, locked in the meat suit. An important part of our bargaining strategy.
While going through old Men of Letters records, trying to figure out a way to cure a demon for the third trial. Sam finds a film reel, which showed a demon chained up in shackles with various binding sigils etched into them, go through a failed exorcism in an attempt to cure the demon.
Crowley is freed from the Men of Letters dungeon, to help expel Gadreel from Sam's body, but only on the condition he needs to wear the binding handcuffs. Crowley also wears a collar that apparently serves the same purpose as he can't leave his meatsuit with it on.