Basic Dungeons & Dragons is a a tabletop roleplaying game. Specifically, it's how Original Dungeons & Dragons got renamed/rebranded around 1977, before TSR introduce Advanced Dungeons & Dragons. The added distinction Basic was used to indicate that the original version would be developed further towards a younger audience, for which it got a horizontal structure, with rules sets that would cover a number of levels of play, while AD&D got a vertical structure, where a rule book would cover all the information in depth for a specific a limited number of characters types, with other character types being covered in different books.
Rules sets[]
Thus, Basic Dungeons & Dragons got divided into Basic Rules, Expert Rules, Companion Rules, and Master Rules, which added ever more complexity to gameplay. Beyond that, Basic D&D covers what happens past the maximum character levels, with the final rules set: The Immortals Rules.