You must either buy a prepaid gift card at a store, or pay on Nintendo’s virtual store with a credit card on the Internet, to be able to subscribe for a certain number of months.
This service allows access to certain NES (Nintendo Entertainment System) and SNES (Super Nintendo Entertainment System) games playable on the Nintendo Switch console.
You must access this service to save game backups to Nintendo’s servers. When they are saved at Nintendo, it makes it easier to share them between consoles, and it makes it possible not to lose its progress in case of breakage or loss of the console. More information here.