Since its earliest release, Java has touted itself as a "write once, run everywhere" programming language. The idea was that a programmer could develop an app in Java, have it compiled down to bytecode, and become an executable that can run on any platform, regardless of operating system or platform. It was able to do so in part by a runtime known as the Java Virtual Machine, or JVM.
To Java's credit, the JVM was (and still is!) an incredibly fine-tuned runtime that abstracted away a computer's underlying hardware. While Java as a programming language survives to this day, it is often viewed as cumbersome, stodgy, and representative of an outdated approach to implementing solutions.