Fine-tuning means creating ("training") your own AI with your own data. The idea is that you give the AI model many examples (in a "dataset")
so it learns from you and is then excellent at addressing your use case. The is the best way to achieve state of the art results in machine learning.
You don't necessarily need to spend too much time on your fine-tuning dataset as modern AI models can be fine-tuned with few examples. For example you can reach great results with only 500 examples.