When you do tasks on Earn.com, you earn bitcoin. This means your earned balance is stored in bitcoin (BTC). However, your balance is displayed in US dollars (USD).
Because the exchange rate between USD and BTC changes over time, your displayed USD balance will change over time even as your BTC balance remains constant.
Please note that once verified you can cash out your balance at any time to an exchange like Coinbase. From an exchange, you can then withdraw your money to your bank account in local currency, which means you are not exposed to either the upsides or downsides of bitcoin volatility.