[1] In principle, it operates like a see-saw, with the balance point located closer to the counterweight, which allows the end of the arm with the camera to move through an extended arc.
Typically a jib permits the camera to be moved vertically, horizontally, or a combination of the two.
[2] Modern jibs are normally modular and can be assembled in various lengths.
A jib can be used for getting high or low shots which are difficult for a hand-held camera operator to get, or shots which need to move a short distance horizontally or vertically, without the expense and safety issues of putting a camera operator on a crane for a crane shot or laying track for a camera dolly.
It supports the camera and enables remote pan/tilt functions with focus/zoom control.