The Time Value of Money
A dollar today is worth more than a dollar tomorrow — and that single sentence, made precise, is the foundation beneath every price in finance. Discounting future cash flows back to their value today is the operation you perform, explicitly or implicitly, every time you value anything: a bond, a stock, an option, a company, a mortgage. Master this one idea and most of finance becomes arithmetic on top of it.
A dollar today is worth more than a dollar tomorrow — and that single fact, made precise, is how everything in finance gets valued. Present value, future value, discounting, and the discounted-cash-flow method that prices anything with a future payoff. (Educational, not financial advice.)