Sibling comment from discordance has a more accurate description of RAG. There's a longer description from Nvidia here: https://blogs.nvidia.com/blog/what-is-retrieval-augmented-ge...
> Given the prompt “When did the first mammal appear on Earth?” for instance, RAG might surface documents for “Mammal,” “History of Earth,” and “Evolution of Mammals.” These supporting documents are then concatenated as context with the original input and fed to the [...] model
Finding the relevant context to put in the prompt is a search problem, nearest neighbour search on embeddings is one basic way to do it but the singular focus on "vector databases" is a bit of hype phenomenon IMO - a real world product should factor a lot more than just pure textual content into the relevancy score. Or is your personal AI assistant going to treat emails from yesterday as equally relevant as emails from a year ago?