One of movies is called The 39 Steps What do the refer to?

The 39 Steps is a 1935 spy thriller film directed by Alfred Hitchcock, based on the 1915 novel of the same name by John Buchan. The title refers to the 39 steps that Richard Hannay, the protagonist, must climb in order to escape from his pursuers. The steps are located in the Forth Bridge, a railway bridge that spans the Firth of Forth in Scotland.