HELP! what does ''currencies'' mean in this context?

I encountered a sentence related to economics. Here it is.
"The government can provide financial sisstance to poor countries and can shore up interest rates in the United States or, more recently, Greece, by buying assets of foreign governments. These are currencies of power."
I got some problems in the understanding of the word "currencies". So, can anyone give me your insights? And another question is about "shore up" in the sentence. Does it mean "to increase", or simply "to sustain"?
I would be very grateful if you guys can help me out of these.