There is an extensive description of the working (and theory) behind the Wishsong in the book "Wishsong of Shannara".
You can also find out more by visiting the Shannara Wikia which provides a reasonable overview without too many spoilers;
The Wishsong is a magic that is controlled by singing. Brin Ohmsford could use it to dramatically change nature, in her childhood, she
changed the leaves of a tree from Green to Brown in the summer, or it
could be used as a weapon. Jair Ohmsford could only use it to create
illusions. Par Ohmsford at first could only use the wishsong to create
images but it evolved into an extremely powerful weapon, which he
ultimately used to destroy Rimmer Dall and all of the Shadowen. Other
Ohmsford family members have the power of the Wishsong as well, either
in the form to create illusions or as a powerful weapon.
Additionally, the book Ilse Witch also contains some info about how Wishsingers learn how to control their powers.
It should be noted that like most magic in fantasy, the actual powers shown seem to vary greatly between books and tend obey the trope "New Powers as the Plot Demands"