Before it
Retrieval meant BM25 and other term-matching methods.
The problem
Keyword matching misses paraphrase, so the right passage is often invisible to the retriever.
What it introduced
Two BERT encoders trained so that questions and their answering passages land near each other in vector space.
Why it matters
It beat BM25 decisively and made dense retrieval the default first stage in RAG.
What modern models inherit
The dual-encoder retriever at the front of most RAG systems.
What came next
RAG plugged it into a generator; hybrid dense-sparse retrieval followed.