JVM起動時の-Xms、-Xmxパラメータとは何ですか？

java -Xms256m -Xmx2048m

-Xmx1024k
-Xmx512m
-Xmx8g

C:\Users\Admin>java -X
-Xmixed           mixed mode execution (default)
-Xint             interpreted mode execution only
-Xbootclasspath:
                      set search path for bootstrap classes and resources
-Xbootclasspath/a:
                      append to end of bootstrap class path
-Xbootclasspath/p:
                      prepend in front of bootstrap class path
-Xdiag            show additional diagnostic messages
-Xnoclassgc       disable class garbage collection
-Xincgc           enable incremental garbage collection
-Xloggc:    log GC status to a file with time stamps
-Xbatch           disable background compilation
-Xms        set initial Java heap size.........................
-Xmx        set maximum Java heap size.........................
-Xss        set java thread stack size
-Xprof            output cpu profiling data
-Xfuture          enable strictest checks, anticipating future default
-Xrs              reduce use of OS signals by Java/VM (see documentation)
-Xcheck:jni       perform additional checks for JNI functions
-Xshare:off       do not attempt to use shared class data
-Xshare:auto      use shared class data if possible (default)
-Xshare:on        require using shared class data, otherwise fail.
-XshowSettings    show all settings and continue
-XshowSettings:all         show all settings and continue
-XshowSettings:vm          show all vm related settings and continue
-XshowSettings:properties  show all property settings and continue
-XshowSettings:locale      show all locale related settings and continue