Tuning Java/JVMs refers primarily to garbage collection (GC) and memory heap size setting.
With the majority of JVMs, setting heap size involves understanding how the application is using memory, adjusting parameters and re-profiling. Whenever possible, the application should be running close to production loads.
This is often difficult to do, as most real-world loads just are not seen in a lab. Most JVMs feature the ability to set an initial and maximum heap size, although a few also provide for other parameters such as large pages. A large heap size setting leads to garbage collection pauses becoming too long; if heap size is set too small, the application will throw an out-of-memory error and likely crash. Thus GC turning and memory are topics that are closely related and they have an impact on application performance.
Learn more about Java garbage collection, and how to remove barriers to Java scalability by eliminating pause times even at very large memory heap sizes, by downloading this white paper about Java garbage collection.