Part of the ATS functionality is the ability to make and store notes about candidates inside their individual MMA.
To make a note jump into the candidate's MMA:
Click "Edit Participant" located on the top right of the page.
A window will slide in with the hiring stages. Scroll to the bottom of the stages, there you will see "Participant Notes" (See Below).
Once you have written your note(s), they will appear on the right hand side of the candidate's MMA under "notes."