To mind or take charge of a location during the time in which it is unattended by another. Don't worry, honey, I'll man the fort at home until you get back from the grocery store. The entire editorial department left the office early for their Christmas celebration, leaving just a couple of interns to man the fort for the rest of the day.

To launch or participate in a protest against some group, organization, or policy one disagrees with. People around the country are manning the barricades following the president's most recent policy announcement. I would be willing to man the barricades if some giant corporation ever tried to buy out our local businesses.