Interest rates are one of the most important drivers for the value of money over time. Interest is the price you get (or the price you pay) for money and it accumulates over time. If you have money today, you can earn that interest. If you don't get that money until a future date, that interest is lost potential that you could have earned. That's why money you get in the future is worth less than money you have now.
Choose whether you're looking for a future value on money you have now, or today's value of money that you'll get in the future. Then enter the amount, interest rate (leave this at 1% if you aren't sure), and the number of years.