When you update your credit card information, you may see the amount of $1 (or £1, €1, etc. depending on the currency where you live) on your statement. You will not be charged this extra amount.
When you buy a book from Kobo for the first time, or if you change the credit card linked to your Kobo account, this extra amount is used to verify your new or updated payment information. You will not be charged this extra amount.
The line on your credit card statement will have the word Kobo in it, possibly as Kobo Internet, WP Kobo, or something similar, depending on your type of card. It may remain on your statement for up to 15 days, after which it will be removed.