What is "Wishsong" in the novel, The Wishsong of Shannara?

The wishsong is a bit of Elven magic. Brin and Jair Ohmsford were born with the wishsong as a result of their father, Wil, having used Elven magic on a quest when he was young. Brin can manipulate nature with her wishsong, while Jair only has the ability to create illusions. Both think the magic is mainly a toy until the day the Druid Allanon asks Brin to use her wishsong to save the Four Lands from dark magic.