RAG Explained: How AI Models Answer Questions Using External Documents

Retrieval-Augmented Generation (RAG) is a technique that allows large language models to answer questions using documents they were never trained on, without any retraining or fine-tuning. First introduced in a 2020 Facebook AI Research paper by Patrick Lewis and colleagues, RAG works by searching external documents for relevant passages and feeding them as context to the model before it generates a response. The approach addresses two core limitations of LLMs: knowledge frozen at training time and no access to private or proprietary data. A typical RAG pipeline involves chunking documents, converting them into vector embeddings, storing them in a vector database, and retrieving the closest matches to a user's query at request time. Research has shown that retrieval quality matters significantly, with fewer but more relevant chunks generally outperforming larger, poorly ranked result sets.