Probably the embeddings API. This guide is what helped me understand the concept https://github.com/openai/openai-cookbook/blob/main/examples...

tl;dr is that you can pre-process each chunk of your database and use embeddings to quickly look up which chunk is most similar to the user's query, and then prepend that chunk to the user's query before giving it to GPT, so that GPT has the relevant context to give an answer.