Joint actors are used to connect two rigidbodies (character controllers are also supported). For instance, by using a distance joint you can create a spring connection between two objects. By using hinge joint you can create a door.
To make joint work you need to add it as a child to the first rigidbody and link the second one to the Target property. Joints support breaking after a certain force applied. Broken joints are not used anymore and should be removed from the game.