Back-slashes are used by Windows (only) for paths. All Unix based systems (Linux, OS X, etc) use forward-slashes. C++ uses back-slashes as escape character (e.g. \n = new line, \t = tab, etc) and it understand forward-slashes for all kind of paths just fine, thus you should never use back-slashes for paths in C++.
"Portable" means that it can be used on any system.