If the credit card setup is not set up to never require the security code, you will be asked to enter the security code when processing certain credit card transactions. The screen displays the masked credit card number (it displays the last 4 digits of the credit card number) and will ask for the security code. The security code is either three or four digits long, based on the credit card type.
You will need to ask your customer for the security code, also known as CVV code, when you are required to enter it. You are not allowed to store the security code for a credit card in a database or on paper. If you do store the security code, you are violating PCI Compliance and therefore could be liable for fraudulent charges.