There are multiple ways of annotating IDA databases: renaming, commenting, or adding bookmarks. However, sometimes there is a need for general notes for the database as a whole, not tied to specific locations.
The database notepad is an input box which can store arbitrary text within the database, so you can add your notes and thoughts there instead of using a separate software.
It can be opened using the menu View > Open subview > Notepad.
Article Link: Igor’s tip of the week #81: Database notepad – Hex Rays