Guanyin is often shown as having many heads and arms. Since this bodhisattva has vowed to save all sentient beings, more heads and arms could help in saving them.
According to one story, despite her efforts to save all beings, Guanyin saw that she could not save so many of them. In realizing this, her head split into eleven parts.
But Amitabha, the Buddha who is master of Sukhavati (the heavenly realm to which Guanyin is connected), helped her by turning the eleven pieces of her head into eleven new heads, so she could better hear the cries of all who are suffering (and thereby try to save them).