Project a starting balance and regular contributions forward at an assumed rate of return, and see how much of the difference is actually your own money versus growth.
A lump sum is hard to picture. Using a common rule of thumb, withdraw a fixed percentage of the balance each year, here's roughly what it could pay you, in today's purchasing power, plus how much you'd need to contribute to hit an income goal of your own.
Interest compounds monthly at the flat annual rate you set above, with your monthly contribution added at the end of each month: a simplification of how real markets behave, which don't move in a smooth, constant line, some years are down, some are sharply up. This also doesn't account for taxes, investment fees or expense ratios, an employer 401(k) match, annual contribution limits, or increasing your contribution over time (say, with raises). Treat the projected balance as a rough, useful illustration of how compounding and time interact, not a guarantee or a financial plan.
It depends on what you start with, what you add each month, the return you assume and how many years there are. The calculator compounds monthly and shows how much of the final balance you put in versus what it earned, which is usually the most surprising part.
A guideline that withdrawing about 4% of a portfolio in the first year of retirement, then adjusting for inflation, has historically lasted around 30 years. The calculator uses the withdrawal rate you set to turn a balance into monthly income.
More than it looks, because the earliest contributions have the most years to compound. The cost-of-waiting panel shows the balance you would give up by starting later with the same monthly amount. How retirement savings work covers accounts and matches.