A peace bond is a very visible lock or tie on a realistic looking weapon to make clear to security and to police that it is not intended to be used for violent purposes.

Costuming frequently comes in the form of characters with weapons, and in many cases, these weapons look quite real. Convention security prefers that these items be peace bonded. The peace bond will usually come in the form of a zip tie over the trigger such that it couldn't be pulled, or holding a bladed weapon in the sheath.

Attitudes about peace bonding are far more relaxed at Dragon*Con than at other conventions: it's perfectly acceptable for members in costume to have un-bonded weapons such that they may pose with them. Some conventions are strict enough to require even the most obviously fake weapon to be bonded. If you are going to costume with weapons, please remember to be respectful with them.