Short answer
The Public Interface lets you create an embeddable chatbot that answers from one Index you choose.
Full explanation
Use the Public Interface when you want visitors to ask questions against a single Index without giving them access to your full workspace. You design it, set basic behavior, and then paste a short embed code into your site. The interface uses only the currently selected Index and shows a live preview while you configure it.
Tips
Keep the target Index clean and well organized to ensure high quality answers.