Power commodities/cargo are the things you carry from one system to another in order to fortify, prepare or control it for your power. They are available through the Power Contact screen under Contacts (depending on the system you're in and therefore the actions available, of course) and have names like Torval's Deeds or Alliance Legislative Contracts, and yield one merit per ton delivered. Starting at rating 1 you can deliver 10t every half hour, this number typically increases as you rise in rank.
If you haven't already, read Barking Mad's
Powerplay guide and LaserusKI's
thread as these provide some really useful and easy to understand information regarding Powerplay.
Hope that helps.