Choose an amount you could realistically spare each month, the age you're starting, and the age you'd stop. The calculator projects what that habit could grow into at an 8% average yearly return, and shows how much of the result came from compounding rather than your own deposits.
Investing a fixed amount on a schedule is called dollar-cost averaging. You buy more shares when prices are low and fewer when they're high, so you never have to guess the perfect moment to invest.