Present Value...
...or The Key To The Universe.
OK, not really, but the first few readings focus on this simple equation: compound interest, and how to make it flow in both directions.
Basically, it lets you take money flows for any period into the future and compare them. One of the obvious uses is to compare projects using something called Net Present Value. The initial investment (and any subsequent investments) are negative cash flows, while the returns are positive cash flows. Discount them back to today, and see what the total are. Then, pick the project that has the highest value.
The CFA also teaches something called the Internal Rate of Return, or the effective interest rate that a project's investment would appreciate at over the life of the project. It's a more complex calculation, and interestingly, because it's much more sensitive to assumptions. It's completely unclear what value this calculation is, since we're told to ignore it when it disagrees with Net Present Value.