Java settings
Here you can set the global Java settings
Java memory settings. For explanation, see: Increasing Java's memory allocation
This is where the settings for the Java runtime live, like the location of the runtime and any Java arguments to use.
Auto-detect will check your computer for all java versions and show you a list of them, the best one on top.
Test can be used to test the selected Java runtime along with your memory settings and JVM arguments without starting the game.
Self explanatory.