Your security code — also known as your card verification value (CVV) — is usually visible on your physical credit card (there are exceptions, like the Apple Card).
With Visa, Mastercard, and Discover credit cards, your CVV is a three-digit number shown on the back of your card. American Express security codes, sometimes known as card identification numbers (CID), can be found to the upper-right of the credit card number on the front of the card.