BASIC is a programming language, and Minecraft is a video game. If you used a Commodore 64 or Amstrad CPC464 back in the 1980s, you probably have some vague familiarity with the language. Anyway, someone has built a BASIC interpreter in Minecraft. It doesn't work very well, and you'll probably never want to actually use it, but the fact that it exists is its own reward.
It's the work of one SethBling, who does a great job of explaining what it is and how it works in the accompanying video. It's made in vanilla Minecraft, and utilises hundreds (or maybe thousands) of command blocks. You can download the scripting here, if for some reason you actually wanna use it, but I feel like it's better just to watch, and to marvel at what's possible.