A PlayStation gift card, or so-called PlayStation Store gift card, is a prepaid virtual payment method that allows you to top-up your PSN account with money. It functions as a form of payment in the PlayStation Store to buy games, downloadable content (DLC), movies and TV shows, music and more!
This is because when you redeem a PlayStation gift card, the funds are automatically added to your PSN wallet and thus if available can be viewed in one of two places: Your PlayStation console or on the website for the PlayStation Store itself. You can then use this money to buy things in the PlayStation Store instead of a credit card or other methods of payment.