How compound interest works
Compound interest means you earn interest on your interest. In year one you earn interest on what you put in. In year two you earn it on your money and last year's interest, so growth speeds up over time.
Time does most of the work. Starting earlier, even with small amounts, often matters more than paying in larger amounts later.
Savings vs investing
Savings accounts pay interest at a set or variable rate. Investments don't pay a fixed rate: their value goes up and down, and the rate you enter here is just an average assumption, not a promise.