The Notes pane displays the notes or comments added to the document and allows users to add their own notes. Use this area to communicate with other users about the document. The notes can include details about the document, information on how it was created, reasons to use it, queries about the data displayed, a back-and-forth conversation about designing the document, or anything useful to the users of the document. The document cache is not invalidated when notes are added or modified.
The Notes pane contains:
Users that have the "Edit Notes" privilege can edit notes as described below:

The comments written get saved when saving the report or document for which they are written and can be viewed by other users who have access to that report or document.
The 'Notes' section can be accessed from the view menu, the object browser window, and the toolbar section on the top.
For more information, refer to Report Services Document Creation Guide.