Property groups enables documenta categorization and search functionalities to your own parameters ( grouped conceptually ).
OpenKM comes by default with a property group example, go to administration an register my_groups.cnd ( it's on jboss_home path ).
Take a moment, when a property group is registered can not be unregistered, be carefull with it, can be increased but not deleted.
On faq section it well explained how to create property groups:
http://www.openkm.com/FAQ.html take a look at "How can I create my own property groups? "