The yorishiro is an object into which a kami is invited to descend and in which it temporarily lodges, the vessel that gives a formless deity a place to be present.
A place for the formless
The kami are not, in the older understanding, bound to images but are formless and roving, and worship requires that a deity be drawn to a particular place and given something to occupy, which is the function of the yorishiro: a tree, a rock, a pillar, a branch, a wand or another object into which the kami is invited to descend so that it can be present and be worshipped. The object is not the deity but its temporary seat, and once the kami has descended into the yorishiro the space becomes sacred and the rite can proceed.
From temporary seat to permanent body
The yorishiro is often temporary, the kami invited into a branch or a wand for the duration of a rite and then sent off again, but the same logic underlies the permanent shintai, the hidden object enshrined as the fixed seat of a kami at the heart of a shrine, so the yorishiro and the shintai are the passing and the permanent forms of a single idea, the giving of a lodging to a deity that has no form of its own. This conception explains why so many natural things, a great tree marked with a rope, a rock, a mountain, are treated as sacred in Shinto: they are or may become the yorishiro into which a kami descends, and the object is honoured for the presence it holds rather than for itself.