If you buy a currency and sell another in the forex market you will have to pay or receive the interest difference between the 2 currencies.
The "Interest Strategy Calculator" will show you whether to buy or sell a currency pair when you wish to RECEIVE interest and how much the currency rate can fall until you start losing money.
If the currency rate runs in the wrong direction (e.g. you bought EUR/HUF and the rate falls) you lose money.
If the currency rate runs in the "right" direction (e.g. you bought EUR/HUF and the rate rises) you make additional profit to the interest payments.