What is an impression on Facebook advertising?

I would like to advertise on Facebook and I am getting confused. I know what they mean by bid per click; however, and I don’t mean to sound ignorant, I don’t know what an impression is. Could somebody explain to me. I am just starting my store out and I don’t want a big monthly expense. Which allows more viewers to see my add at a reasonable monthly Cost. So I guess I have two questions. Help, please.