All that is gold does not glitter,
Not all those who wander are lost;
The old that is strong does not wither,
Deep roots are not reached by the frost.
From the ashes a fire shall be woken,
A light from the shadows shall spring;
Renewed shall be blade that was broken,
The crownless again shall be king.
Refers to the sword known as Narsil. Narsil was destroyed (broken in two pieces; in the Peter Jackson film, multiple pieces) in the downthrow and apparent killing* of Sauron at the beginning of  The Fellowship of the Ring . 
 
 Elrond, fulfilling the prophecy, reforges Narsil as Andúril (“flame of the west”) 
 Tolkien may have been inspired in his creation of Narsil/Andúril by Gram of Northern mythology, which originally belonged to Sigmund, was shattered in battle, and was reforged for the use of his son Sigurd after his death. 
 
 *Sauron, it transpires, wasn’t killed; as Gandalf discovers to his horror shortly prior to the events of  The Lord of the Rings , he was simply cut off from the life force that he had tied to the One Ring and disappeared, returning many centuries later until he was finally destroyed.