The term refers to the Nintendo Switch Package file format used for digital software distribution. In the enthusiast and homebrew community, NSP files are often used to manually install updates on consoles running custom firmware (CFW).
For most players, the easiest way to update is through the official Nintendo Support method: Connect your console to the internet. Highlight the game icon on your HOME Menu. Press the or - button. Select Software Update and then Via the Internet . Understanding NSP Updates dr kawashimas brain training switch nsp update link
Updating your game is highly recommended to access these major additions: The term refers to the Nintendo Switch Package