Portable code is easy to move to other platforms -- either other compilers or other operating systems, with a minimum of changes needed to facilitate the migration.
Ideally this is achieved by only making use of standards (types, functions, definitions that are defined in system-provided header files, where on another system those header files might be modified by the compiler / OS vendor to be correct for that system).
Sometimes use of standards is insufficient, requiring use of (generally ugly) #ifdef's sprinkled throughout the code.
My statements assume a C/C++ like language. Many other languages are more intrinsically portable.
