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.
For information about how to get a correct Java version, see: Using-the-right-Java.
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.
Pre-launch command runs before the instance launches and Post-exit command runs after it exits.
Both will be run in MultiMC's working directory with INST_ID, INST_DIR, and INST_NAME as environment variables.
Wrapper command allows running java using an extra wrapper program (like optirun on Linux, for use with optimus graphics laptops)