NotebookLM is a tool from Google, using its Gemini engine, that allows users to create "notebooks" of text files, PDFs, websites, notes, and more, and "chat" with them to draw insight from all these sources at once.
It uses a principle called Resource Augmented Generation, or RAG, which basically means that, instead of the chatbot trying to draw on its training data for its response to your query, it sticks as closely as possible to the source material you provide it. This dramatically cuts down on "hallucinations," but if the sources you give it are unclear, contradictory, or contrary to reality NotebookLM's replies may be as well!
On the free tier, a user can upload up to fifty sources per notebook, each of which can have up to 500,000 words (or 200 MB in size). One can upload PDFs, text files, web pages, Google Docs and Sheets, YouTube videos (from which it extracts the transcript), and more. NotebookLM will extract the text and work from that; it will not update itself automatically if a website or Google Doc changes.
Files you put in a notebook stay within your Google account and are not made public or added to training data; however, as always one should avoid putting sensitive information on the internet if one can. You can share a notebook with other Google account holders, but they cannot be made truly public.
One feature of NotebookLM that has received a lot of attention is its Deep Dive Conversation feature, where one can create an audio file that simulates two hosts discussing a topic based on your sources. One can even have a live conversation with the "hosts."