Exercises, routines, workouts, sets - all of this may have some additional information as notes. The main non-obvious feature of notes in this application is their view mode. The idea is to show all relevant notes at once.
Some notes are highlighted in color. Those that were made as a comment to the exercise settings (information) are marked in red. Those related to exercises and sets within the workout routines are highlighted in blue.
To open your notes in preview mode (where possible), tap the More icon to open the menu, then tap the Read Notes menu item.