Political capital ("PC") is NationBuilder's currency system. In nonprofit, business and government editions, it's called "social capital" by default, and all nations can customize the name of their currency.
Political capital is earned through scores of activities, such as RSVPing for events, mentioning a nation's broadcaster(s) on Twitter, recruiting new signups to a nation, and having many Twitter followers who are also members of a nation. You can customize the defaults for political capital in your control panel under Settings > Defaults > Political capital.
Political capital is a useful metric for quickly identifying influential and active members of your nation. Under the "People" section of your control panel, you can sort any of your lists, tagged groups, or saved filters by their amount of political capital (Watch Kathy and Adriel discuss currency search options on this clip from NationBuilder Live). You can also use political capital and public leaderboards to promote your nation's supporters.
More on political capital:
in this clip from NationBuilder Live, C.J. explains how to use leaderboards: