The Silvers Shoes are magical foot-wear belonging to Nessarose Thropp.
According to the book, Nessarose was born without arms and therefore constantly in need of assistance from someone else. They were forged by the Quadling Turtle-Heart and given to her as a gift when she entered Shiz. After her sister's departure, Glinda magicked the shoes to assist Nessarose.
The power in the shoes made Nessarose able to support herself without assistance by giving her the sensation of "phantom arms" and creating a sense of balance. Without her sister's support, the shoes gave her the confidence she had lacked in her earlier life, eventually leading to her succession as the Eminent Thropp. After Nessarose was crushed by Dorothy's house, Glinda gave the shoes to her along with a charm that prevented their removal.
In the Wicked Witch of the East song, Elphaba looked in the Grimmerie and did the shoe spell on Nessa's shoes. They turned ruby-red and allowed Nessa to walk on her own.
Silver vs. Ruby Red?
In the book The Wonderful Wizard of Oz, the Wicked Witch of the East's slippers were of silver. Due to technical complications surrounding filming reflective substances such as gold and silver, the silver slippers were retconned in the film The Wizard of Oz as the now-popular ruby slippers.
In the novel, Maguire addressed this issue in that, originally, they were silver with glass beads which, after being enchanted, would reflect various colors (one of them being ruby red). In the musical and its various adaptations, the shoes are silver (as a nod to both books), which Elphaba then enchants, after which they are ruby red.