You might be referring to the game's file format or digital distribution. NSP is a file format used for Nintendo Switch games distributed through the Nintendo eShop. When you purchase Taiko no Tatsujin: Rhythm Festival digitally, it is downloaded as an NSP file.
The gameplay involves tapping drums in time with music and on-screen prompts. Players use a taiko drum controller (which can be purchased separately for the Switch), which comes in a variety of colors and designs. The game features a variety of songs from different genres and includes both popular songs in Japan and classic video game soundtracks.
The city is divided into five main areas that house different modes: