A token is the chunk of text a language model actually processes — roughly a word or word-piece. Models read and write token by token; context windows, speed, and API pricing are all measured in tokens.
Tokens are the currency of LLMs. Knowing this explains why long documents cost more, why models sometimes split words oddly, and what '128k context' really bounds.
The sentence “AI moves fast” is about four tokens; a full novel is hundreds of thousands — which is why it cannot fit into one prompt.