A shimenawa is a straw rope with white zigzag paper strips (gohei). It marks the boundary to something sacred and can be found on torii gates, around sacred trees and stones, etc. A rope similar to the shimenawa is also worn by yokozuna, the highest ranked sumo wrestlers, during ritual ceremonies.

(these words from this site: http://www.japan-guide.com/e/e2059.html. )