Signing up for a PayPal account allows you to pay using your bank account, credit card, or PayPal balance (in most cases gift cards are not accepted).
If you don’t have a PayPal account, you can still complete your purchase with PayPal using “Guest Checkout”. After clicking “Checkout” on Bandcamp, look for the “don’t have a PayPal account?” option on PayPal’s details page, then enter your credit card information. Unfortunately, PayPal does not allow Guest Checkout in 100% of cases — sometimes it’s because of your geographic location or past purchases, and other times it’s because of incorrect settings in the artist’s PayPal account. If you don’t see the option, stress not. You can still complete your purchase by signing up for a PayPal account.