OSM Notes is a core feature of the OpenStreetMap.org website for placing shared notes on the map to assist in mapping/editing OpenStreetMap. It is not intended for general discussion or storing information not otherwise used in OpenStreetMap; for example, “I was here on Saturday” or “There was a crime at this location”. Notes allows for two way communication, with the ability for a mapper to ask for more details if necessary. Although it is not necessary to be logged in to submit (or comment on) a note, it is beneficial to do so. Logged-in users will receive an email if comments are added to a note or the issue is resolved.
Actually it is not necessary, the function works above any map data, but if you are willing to edit OSM maps, it is understandable. It is also possible to open another map in overlay mode to compare and for better placement of your new notes.
Start OSM Notes function from Menu > More functions > Notes (OpenStreetMap). The function's dialog window is divided into a few sections:
It may require zooming your map out to see any notes as there are usually not many of them. The data are loaded from a server when you stop moving with map so it may take a while till they are displayed.