Its at the top of the touch screen, it's a little pencil you can view during your games just like notifications and friends. Say you need to write down a code or something to remember later, this was added for that to avoid those pencil and paper runs for those kind of things.
I've used it to write down the Scarecrow song for OoT 3D, jotted down combos to practice/try for SSF43D, I've doodled in them, wrote down important stuff to remember i.e. shopping lists and directions, and a few other things.
P.S. To access the notes click on the pencil icon at the top of the home menu.
An apology for the devil: It must be remembered that we have only heard one side of the case. God has written all the books. -George Herbert