It's just a pre-made group you can use to put things into. You can make your own groups as well.
The only real difference between it and the Clipboard Old/New group is that the Old/New group optionally (but enabled by default) will move clips from New to Old to the recycle bin over time, to keep things tidy. No such moves will take place in the My Notes group.
But again, it's just a premade group you can put clips into. You can make your own groups that work identically.
Do note that there is a button at the upper left of the Memo panel for creating a new clip (or you could call it "note") in whatever group you are currently in, manually.
There was a point when I was considering making CHS a full-fledged note taking application, so some of the features can be seen in that light. Having said that, I'd advise against people really trying to make heavy use of it as a note taking app, simply because there are better tools for that.