In the blog post, a new data structure called the Document Summary Index is introduced for LlamaIndex, designed to improve retrieval performance over traditional semantic searches. This approach leverages a Large Language Model (LLM) to extract summaries from documents, storing both these summaries and text chunks in a Document Store. During query-time, relevant documents are retrieved based on their summaries using LLM-based or embedding-based retrieval, offering a more flexible and context-rich method than traditional text-chunk retrieval. The Document Summary Index allows retrieving documents at a more comprehensive level, providing more context than simple top-k text-chunk retrieval without the complexity of keyword tagging. This method enables more efficient use of LLMs by first evaluating document summaries for relevance, enhancing retrieval efficiency and reducing latency. The post also showcases an example using Wikipedia articles, demonstrating how the index synthesizes responses through both high-level and lower-level APIs, and hints at future developments in autosummarization and LLM-based retrieval.