Why people say that Java is 'write once and run anywhere' language?

 

You can write Java code on Windows and compile it in Windows
platform. The class and jar files that you get from Windows
platform can run as it is on Unix environment. So it is a truly
platform independent language.

Behind all this portability is Java byte code. Byte code generated by
Java compiler can be interpreted by any JVM. So it becomes much
easier to write programs in Java and expect those to run on any
platform.

Java compiler javac compiles java code and JVM java runs that code.