Games were typically built using Java Swing or Abstract Window Toolkit (AWT) for 2D graphics. As the technology matured, libraries like allowed developers to tap into the computer's graphics card, enabling hardware-accelerated 3D graphics within a browser window.
The "Java Applet" was a small application designed to be embedded within a web page. When a user visited a site hosting a Java game, the browser would download the small bytecode file, and the JVM plugin would execute it. This was a paradigm shift. Suddenly, developers didn't need to port their games to Windows, Mac, and Linux separately. They wrote it once in Java, and it played everywhere.