From LQWiki

POSIX (Portable Operating System Interface) is a standard that describes a standard interface which ensures that software source written on one POSIX compliant operating system can be recompiled on another POSIX compliant operating system with little difficulty.

Naturally, this is not always so cut-and-dried, but the use of standardized functions makes porting software much easier than having to detect idiosyncrasies in operating system implementations of standard library functions, for example.

Many Unix and Unix-like operating systems are POSIX compliant, however today greater weight is given to the more open Single Unix Specification published by The Open Group, since the text of the SUS is freely available, while one has to pay for the text of the POSIX standard.

The great thing about standards is that there are so many to choose from. - Old Hacker Saying