What Is Bert Fine-tuning?

What Is Bert Fine-tuning? “BERT stands for Bidirectional Encoder Representations from Transformers. … As a result, the pre-trained BERT model can be fine-tuned with just one additional output layer to create state-of-the-art models for a wide range of NLP tasks.” That sounds way too complex as a starting point. What is fine-tuning in deep learning?