Debit cards typically have two different types of numbers on the front. The first is your account number, which is unique to you and identifies your specific card. This number usually consists of around 16 digits. The second set of numbers, which usually appears after or beneath your account number, is your debit card's routing number. This nine-digit code tells banks where to route money when it's being transferred into or out of your account.
The first line on a debit card, then, is generally your account number. This can be followed by the routing number on the second line. Some cards may have additional lines for things like expiration dates or security codes, but these are typically found on the back of the card instead of the front.