They are the notes kept on/about your file by the VO at Buffalo. It has a set format, so you will see info regarding when your application was received, when it was opened, when they requested/received your criminal checks, medical results, etc. Plus at the end there is a section for "hand written" (though they're actually typed) additional notes, where you tend to see things like, "Relationship appears genuine, no need for interview." If you're going to Quebec, it will also state if your CSQ has been received by Buffalo.
Mine showed me that someone at Buffalo was bs-ing the staff at my husband's MP's office when they kept telling them that my criminal background check hadn't been received. They were telling them that in October, but the notes showed they'd requested and received results for the check in July! It also showed that in July my file was "sent to the put-away pile" and then there had been no updates since then (i.e. no updates to the file between July and Nov.). This helped me when I contacted Buffalo afterward trying to get them moving on my file again.
What you can do once you have for your GCMS notes is use that info when contacting Buffalo - whether it be directly or through an MP. So if you see Buffalo doesn't have your meds or your payment you sent in months ago you can contact them and state this.
Basically information is power in this situation, in that instead of emailing and saying "hey why is it taking so long" you can email and say, "hey why does it say you haven't touched my file since Sept. 15th, on which date it appeared you had all the info you needed to move forward with me file?" Or something along those lines.