http://mochida.cc/archives/tag/%E6%9D%91%E6%AD%A3

Shimenawa (標縄・注連縄・七五三縄, literally “enclosing rope”) are lengths of braided rice straw rope used for ritual purification in the Shinto religion.

A space bound by shimenawa often indicates a sacred or pure space, such as that of a Shinto shrine. Shimenawa are believed to act as a ward against evil spirits and are often set up at a ground-breaking ceremony before construction begins on a new building.

They are also used to mark trees that are believed to be inhabited by spirits called Kodama. Cutting down these trees is thought to bring misfortune.

http://en.wikipedia.org/wiki/Shimenawa