A platform is a combination of hardware and software used to run software applications.
A platform can be described simply as an operating system or computer architecture, or it could be the combination of both. Example: Microsoft windows, LINUX, UNIX and so on.
The every operating system has unique specification to run the application.
The Java program can run on many operating systems therefore it is called as platform independent.
The Java platform is a virtual machine platform or common platform for software to be written for.