Playing a string and at the same time pressing and letting it slide a metal rod along it will cause a a change pf the played pitch. The result can be a soft flowing glissando or also a melody can be played on one string by moving the position of the rod. In this case it is important to know that the string that is played should be at least a third lower than the note that should be heard.
Carlos Salzedo used to use the tuning key to perform this effect, the by him called “fluid sound” in the beginning of the 20^th century, because the tuning keys were made of metal. That is why the most common sign for a metal rod is the symbol of the tuning key, even though nowadays they are often made of rubber.
Of course the metal rod can be used also for other techniques and not only for the fluid sounds. For example,  it can be hit against a string, be moved fast between two strings will result in a type of tremolo, or be used for a glissando . It can be also used in a  more percussive way, by gliding along the tuning pins.

For more effects, check out the video.