Khoj is a personal AI app that allows users to chat offline with their second brain using Llama 2, accessing information from the internet and local documents. It can be self-hosted or used as a cloud service, and has features such as semantic search, automation, and custom agents. The app integrates with various platforms, including Obsidian, Emacs, and WhatsApp. Khoj is open-source and aims to provide a private and customizable AI experience.