Fine-tuning
Fine-tuning means training an existing AI a little more on your own examples, so it learns your style.
In a simple example
You show it a thousand of your favourite product descriptions, and it learns to write like you.
Why it matters to you
It is slower and costs more than writing a good prompt, so most people never need it.
Do not confuse with
Fine-tuning changes the AI itself. Prompting only changes what you ask it. Start with prompting.