The idea
A free-text notes field on each source in your library. Write whatever you want: a summary in your own words, a reminder of why you saved it, a flag for a specific page number. Notes are private to you and live with the source.
Why it matters
Researchers annotate their sources. That's a basic part of the workflow. Too often, those annotations live in a separate app, a spreadsheet, or a sticky note on the desk. They should live next to the source, in the same tool where you're citing it. When you're deep in a literature review and you need to remember why you saved a paper three weeks ago, a quick note beats re-reading the abstract.
Where we are
This feature shipped. In the Sources library, open a source and use its Notes tab to add or edit a note. The editor's Sources panel shows your notes read-only while you write, and Save as note in the reader's Ask tab adds an answer to them. Library search doesn't look inside notes yet; use tags for anything you want to filter by.