Income account is a chart of account in your QuickBooks Online which helps you track income from selling products in your Shopify store.
For instance, let's say you have an order in your store (As shown in the below image), then income account will be updated with the addition of 60 USD once this order has been synced to QuickBooks Online.
Please note that it considers only the income obtained by selling the products, it doesn't include the income obtained by Shipping charges or so.