Albert Einstein reportedly called compound interest the "eighth wonder of the world," stating, "He who understands it, earns it; he who doesn't, pays it." Unlike simple interest, which is calculated only on the principal amount, compound interest is calculated on the principal plus all of the accumulated interest from previous periods.
The most powerful variable in this calculator is not the "Monthly Contribution" or even the "Interest Rate"—it is the "Investment Period." Because compounding is exponential, the last few years of an investment often generate more returns than the first 20 years combined. This is why starting early (even with small amounts) is statistically better than waiting until you have a larger salary to begin investing.
When entering your "Interest Rate," it is helpful to use historical benchmarks. The S&P 500 (the US stock market) has historically returned about 10% annually before inflation (or 7% after inflation). High-yield savings accounts typically offer 3-5%, while checking accounts offer close to 0%.