From LQWiki

Portability is the idea that code can be easily transfered between operating systems and system architectures with little or no changes required. It is often the result of using a significant level of abstraction from the hardware.

For example, assembly code is not very portable as many different system architecture's require different code (and have different instructions), whereas higher-level languages (such as C, C++, Java, etc.) are abstract enough that the same source code can be used on many different architectures through the use of a compiler.