Nintendo Switch Online is Nintendo's online subscription service akin to PlayStation Plus and Xbox Live. For the price of $19.99 per year, you can gain access to a variety of exclusive features — one of them being particularly essential.
To participate in online multiplayer sessions with other Nintendo Switch users, in the likes of Mario Kart 8: Deluxe, Splatoon 2, and many more, you'll need to have a Nintendo Switch Online membership. NSO also offers expansions like the "Happy Home Paradise" DLC for Animal Crossing: New Horizons.
Additionally, cloud saving, an exclusive smartphone app which features the ability to voice chat, and a collection of classic Nintendo titles hailing from the Nintendo Entertainment System and the Super Nintendo Entertainment System — A.K.A. the NES and SNES respectively.
If you're at all interested in the above features, you'll be glad to know that as of now you won't even have to spend a penny to try out Nintendo Switch Online. Nintendo have announced (via @NintendoAmerica) a new free trial period that's available now in the Nintendo Switch eShop. A full week's membership that includes all of the aforementioned features.