She is a Mahavidya, a list of Sakta and Buddhist goddesses, and the sixth listed on the Wikipedia page. (I don't know if that's important.) Here, she is described as the 'self-decapitated Goddess.' What is the legend/myth behind this title?
The legend behind the severed head of Chhinnamasta is described in the Pranatoshini Tantra text. The story goes as follows:
Goddess Parvati was bathing in Mandikini river where she gets sexually excited and as a result, her complexion turns dark. Her two attendants, Dakini and Varnini, ask her for food at the exact same time. The Goddess tries to convince them to wait till they reach home, but due to the constant requests of the two attendants, She severs her head using her own nails and feeds her attendants her blood.
The severed head of Goddess Chhinnamasta, also has two symbolical meanings:
1) Sacrifice of Ego: Since the head is severed by Her own Will, it symbolises the sacrifice of one's ego.
2) Turiya: Turiya is a state where an individual's mind is dissolved. Since Her head is severed, it signifies a state of mindlessness.