Compound interest means that you will receive interest on the initial capital invested as well as the interest you earn/receive/gain during a certain/set/given period.
The app will show how your investment grows over time. In the settings you will direct how much capital you wish to commence your savings with (i.e. a monthly savings scheme), average interest rate per year and the number of years you wish to continue with this plan. The app will then quickly calculate how large the sum will be at the end of your savings plan and how much of those earnings come from the interest.