"The Torii is a gateway arch that signifies the passage from profane to sacred (unclean to clean). When combined with washing one's hands and mouth with water, passing underneath a torii on the way to visit a shrine is an act of sanctification and purification. People that are in an unclean state are not permitted to approach a Shinto shrine as their uncleanliness would defile the grounds. Some shrines can have multiple torii, but there is usually a "first torii" (ichi no torii) that is larger than the others and stands at the entrance to signify entry into the shrine region. Most often associated with Shinto shrines, torii can also be found at some Buddhist temples.
A torii is formed from two upright posts topped by a horizontal kasagi (cap beam) that extends beyond the uprights on either side; beneath the kasagi is a nuki (horizontal tie beam) which is mortised through the uprights and links them together. There can be some stylistic variations, depending on the overall style of the shrine, as long as it follows the basic form.
The true origin of the torii is unknown. There are many theories, but no single one has gained general acceptance. The word “torii” literally means “where the birds reside.” Since birds act as messengers between the spirits and humans in Japanese mythology, it seems appropriate that they should "be" at the boundary between the sacred world of the shrine and the profane world outside."