Notes are arbitrary events that can be attached to various objects in the system. They are immutable and can only be created or deleted, not modified; they're useful for imbuing Buttondown's objects with additional context and history. Here, for example, is a series of notes attached to a subscriber:
Notes are useful in the obvious documentation context, but become powerful when you chain them with automations. For instance, you could set up a system to enroll any subscriber who receives a given note into a particular welcome sequence.