The shintai is the object in which a kami is present at the heart of a shrine, and its defining feature is that it is kept permanently hidden, often even from the priests.
Vessel, not deity
The shintai, the body of the kami, is not itself the deity but the vessel the deity inhabits, enshrined in the inmost sanctuary as the focus of worship and kept wrapped and unseen. It may be a mirror, a sword or a jewel, a stone, or at the most ancient shrines a natural feature, a whole mountain, a waterfall, a great tree or rock. The most celebrated is the sacred mirror of Amaterasu enshrined at Ise, one of the three imperial regalia, held to be the very body of the deity.
Concealment as protection
Because the shintai is the point at which the kami is present, its installation consecrates a shrine and its concealment protects the holiness of the deity from the defilement of ordinary sight. The logic is the reverse of the murti, which is made to be seen, bathed and dressed: the Shinto presence is guarded by being hidden. That so many shintai are natural features, a mountain or a waterfall, is also why so much of the Japanese landscape is treated as sacred, the object of worship being not a made image but the mountain itself.