It defines an interface (POS I X) to a computing environment (POSI X) that is meant to be portable (P OSIX) across operating systems (P OS IX). This interface extends across both user (commands) and program (libraries) space, and allows both source code as well as shell scripts to work in any compliant environment.

Simply, if the program uses POSIX capabilities then it should work on any POSIX-compliant OS.