Fine-tuning is additional training that adapts an existing model to a narrower purpose: a company's support tone, medical terminology, or a specific task format. The base model's general abilities are kept; the extra training shifts its habits.
Fine-tuning explains how one base model becomes many specialized products. It also has limits — it adjusts behavior more than it adds fresh knowledge.
A law firm fine-tunes a general model on its document style so drafts come out in house format instead of generic legalese.