The "-D" prefix in the argument means that it's a system property.
A system property is defined like this in(http://docs.oracle.com/javase/jndi/tutorial/beyond/env/source.html#SYS):
A system property is a key/value pair that the Java runtime defines to describe the user, system environment, and Java system. The runtime defines and uses a set of default system properties. Other properties can be made available to a Java program via the -D command line option to the Java interpreter.