That's your money working for you! It shows the annual yield rate (APY) you earn on your current account balance.
2% for Standard users
4% for Premium and Native users
The rate is fixed and annual, but the good news is that interest is credited to your account every single day.
However, payouts only appear in your account once the accumulated yield reaches €0.01. So if your balance is on the lower side, you won't see a daily transaction, but rest assured your yield is quietly building up in the background until it hits that threshold.