GPT index provides a few key functionalities and data structures on top, using LLMs, to augment things.
You can store documents in multiple indexes, combine them in different ways, and generally provide a lot of ways to get answers to things using your data.
For example, the VectorIndex finds the most relevant document chunks in an index using vector similarity, then feeds that information to an LLM to generate a natural language response using that context information. Or, you can customize the prompt to get the LLM to do whatever you want!
This page has details on each index and what it does:
https://gpt-index.readthedocs.io/en/latest/guides/index_guide.html