LlamaIndex is an open-source framework for building LLM applications by connecting custom data to LLMs. It excels in Retrieval-Augmented Generation (RAG), data storage, and retrieval. It works by ingesting data from various sources, indexing it (often into vector embeddings), and querying it with a language model. LlamaIndex has tools to evaluate the quality of retrieval and responses. It supports AI agents for automated tasks. The framework facilitates the creation of custom knowledge bases for querying with LLMs.