A miniature microphone and transmitting device.”— Fallout in-game description
A bug is a miniature microphone and transmitting device. This item is given to the Vault Dweller after an assassination attempt has been made on the mayor (and proprietor of Darkwaters General Store), Killian Darkwater. As Killian believes that local Casino owner and reputed gangster Gizmo is behind the assassination attempt, the bug is intended to Get Gizmo's confession. The tape recorder can also be used to get the confession.
To place the bug, simply use the Steal skill and drag the bug to Gizmo's inventory.