Notes are free text descriptions of any information you might want to record to keep track of testing goals, why certain entities were created or to remind you of something.
You can add notes in the single adgroup screen and/or the multi-adgroup screen to help you track your testing strategy and plans. A different note text can be used for single-adgroup from multi-adgroup.
Notes can be recorded for campaigns, adgroup or ads by hovering over the symbol next to the entity. When the note icon is grayed out, it implies no notes currently exist. When you add a note, the icon changes color to solid.