Broadly, it means that you won't be able to buy or download new games on the 3DS or Wii U eShop after 27th March 2024, and you also won't be able to purchase in-game content, download demos or free-to-play software, redeem download codes, or add funds to your account on 3DS or Wii U.
In early April 2024, online play and other functionality that uses online communication will end for Nintendo 3DS* and Wii U software. This also includes online co-operative play, internet rankings, and data distribution.