Actually it means nothing and is ignored.
From the Bash FAQ E10::
E10) Why does 'cd //' leave $PWD as '//'?
POSIX.2, in its description of 'cd', says that three or more leading
slashes may be replaced with a single slash when canonicalizing the
current working directory.
This is, I presume, for historical compatibility. Certain versions of
Unix, and early network file systems, used paths of the form
//hostname/path to access 'path' on server 'hostname'.
Also the Unix standards states:
A pathname that begins with two successive slashes may be interpreted
in an implementation-defined manner, although more than two leading
slashes shall be treated as a single slash.
