AR stands for Augmented Reality. On the Nintendo DS (specifically DSi and 3DS families), this refers to a feature using the system's cameras and physical AR Cards.
Place an AR Card on a flat surface, point the camera at it, and the system overlays 3D graphics, animations, or interactive elements onto the live video feed, blending digital content with the real world.
Limited by low camera resolution and the need for physical cards, DS AR was a novelty. It paved the way for more robust AR on 3DS, mobile (Pokémon GO), and Nintendo Labo Toy-Con.