The Shinto Torii Gate is used by the Shinto religions to symbolize the transition from this world to the next. Actual Shinto Torii gates are found by Shinto shrines. They are built on the path to the shrine. If there is more than one gate they symbolize walking into a holier, more sacred area.
The kasagi is the top of the symbol. It is a horizontal, sometimes curved line that represents the division of the finite and infinite world. The hashira are the supporting posts. They hold up the kasagi and allow priests to walk under the kasagi.
This symbol is also found in the Buddhist religion where they also use it to mark their shrines.
In modern times the symbol is found being used in non-religious contexts also. The Marine Corps Security Force regiment and the 187th Infantry Regiment, 101st Airborne Division displays the symbol on their uniforms. It symbolizes a gateway of honor.