I can also see it meaning a Posse comitatus if a state of emergency exists (Hurricane, Big fire etc.) to act as guards or deputies. A county sheriff or a police chief should be able to call for civilian volunteer backup if needed.
It is NOT just a National Guard.
According to U.S. Code 10:246, it’s every able-bodied male aged 17-45 not convicted of a crime. So basically everyone not from Baltimore. :P
It sounded to me like the reason for Posse Comatatus was that people were supposed to be allowed to create their own local military forces (militia) to deal with threats in their area, not have to wait and hope for the Feds to deal.