The fundamental principle of macroeconomics is the same as the fundamental principle of microeconomics--scarcity.
A general glut, an output gap, and involuntary unemployment are interesting and important because of scarcity. Many macroeconomic fallacies result from the false notion that the key economic issue is finding things for people to do rather than effectively allocating resources to produce what people want most. The disruptions caused by monetary disequilibrium are problems because they interfere with the market processes that embody the most effective social response to scarcity.
Frederic Bastiat did a great job in exploding fallacies and explaining how scarcity is really the fundamental economic issue.
The video comes from the Atlas Foundation.
In Charleston, South Carolina, we have the Bastiat Society. I strongly recommend the monthly meetings. Lately, the meetings have been at the Harbor Club.