What does 'write once, run anywhere' mean and how does Java achieve it?

You compile your source once into platform-neutral bytecode (.class files), and that same bytecode runs on any machine that has a JVM, without recompiling.

Java achieves it by inserting the JVM as a layer between your program and the OS: javac targets the abstract JVM instruction set instead of a specific CPU, and each OS ships its own JVM that translates that bytecode into native instructions at run time. The bytecode is the same everywhere; only the JVM underneath differs.