What does portability mean?
It means coding a program in a way so that the same code works in different environments; for example different processors, different operating systems, different versions of libraries etc.
If your code is portable, you should be able to just re-compile it on any new system and it should run without problems.
Why is it important?
The reason it's so important is because non-portable code can cause many problems in regard to maintenance - managing multiple versions, poor readability/understandability of the code to name a few.
