I believe that the parent is talking about how Java, and in turn the JVM, defines its memory model, which describes formally how memory reads and writes occur in the presence of multiple threads of execution. For example, data races are well-defined in Java: Either you read the old or the new value, you'll never read a mix of two values.

Having a well-defined memory model is important when running on infra with very different models, such as x86 and aarch64.