What is the Currency Converter+ cart note?
The cart note is a text that will only appear if your customers are seeing the prices in a currency different from the currency that will be used for checkout.
In order to prevent any confusion to your customers, you can explain the currency change using the cart note feature.
The note will be displayed like this on your shopping cart:

The cart note is only displayed to customers that are browsing your page in a currency different than the checkout currency. That is, it will only appear if the customer currency is not among the currencies you have configured in Shopify Markets.
You can configure the note from the app preferences dashboard page within the section Cart note:

In the text field of the Cart Note, you can add the following text or any text you want:
While the content of your cart is currently displayed in {{user-currency}}, you will checkout using {{checkout-currency}} at the most current exchange rate
Additionally, you can use the following placeholders:
- {{shop-name}}: will be replaced by the shop name.
- {{shop-currency}}: will be replaced by the shop currency configured.
- {{user-currency}}: will be replaced by the customer currency.
- {{checkout-currency}}: will be replaced by the current active currency if Shopify Payments are enabled, or by the shop currency otherwise.
If you don't want to display the cart note, just leave it empty and click save. The cart note won't be displayed if you configured it that way.