In previous posts such as Linux vs Unix and Types of computer architecture (part 1) we've looked at operating systems and types of computer architecture. If you haven't read these previous posts you may want to.
A platform in technology is the underlying OS + Hardware combination that application/s can run on. Bellow are some examples:
Windows Server on x86-64 Hardware
Solaris on SPARC Hardware
Red Hat Enterprise Linux on x86-64 Hardware
z/OS on z/Architecture Hardware