In traditional meaning "writing portable code" means that you create a code that can be compiled and executed without major changes on several platforms (compilers/operating systems). It's important to know how to achieve this if you want your code to be compiled on many platforms. There are several techniques used for this purpose in relation to C++. Note that sometimes "portable" means just "portable across all Unix systems", especially when somebody mentions POSIX standard.
- Mozilla C++ portability guide
- Writing Portable and Safe C/C++ Programs - Nick Urbanik, 2005
- Using the Standard Header Files to Write Portable C/C++ Applications - Darryl Gove, 2011
- Writing Portable C - David Chisnall, 2010
- See undefined behaviour - you should eliminate all UBs from your code if you want it to be portable