A note can be assigned to tracks using the built-in rich text editor. If a track has a note assigned to it, the Note column will show an icon, and hovering the mouse over that cell will display the note in a popup tooltip:
Double clicking the cell will present a rich text editor to edit the note. More information on the operation of the editor can be found in the linked documentation page. As always, you can multiselect + doubleclick to edit many at once.