In present practice a miko is a young woman who assists at a Shinto shrine, and the office as it now stands is a considerable narrowing of what it once was.
The present role
Today the miko performs the ceremonial kagura dance, sells amulets and other shrine items, and assists the priests at ceremonies, typically as a young woman working at the shrine for a period rather than as a lifelong office, recognisable in the white upper garment and red skirt. In this form the miko is a familiar figure at shrines, especially at festivals and the new year, and the role is auxiliary to the priesthood.
From medium to assistant
The office was once far more, for in earlier times the miko was associated with spirit possession and oracular utterance, a woman through whom a kami could speak, so the role reached back to an ancient function of female mediumship at the heart of Shinto and of Japanese religion more widely. That oracular dimension was largely stripped away in the modern reorganisation of Shinto, especially under the state administration of shrines, which reduced the office to its present auxiliary and ceremonial form. The history of the miko thus records a common pattern in which an older and more charismatic religious role held by women, that of the medium and oracle, was diminished into a subordinate and largely decorative one, and the present shrine maiden is the much-reduced heir of a once powerful office.