Any element can have its own tooltip and note. The difference between tooltip and note is that, tooltip will be displayed in the HTML5 simulation, when you move your mouse cursor hovering on the element, while note will not be displayed and it just helps you to remember some information about the element.
In the tools panel, you can see two categories named "Tooltip" and "Note":
You can click the button on the right to input the tooltip for element.
If the tooltip of element is not empty, the tooltip will be displayed in simulation, when you hover your mouse cursor on the element:
Since V3.90, tooltip of element could be set/removed via action Set Tooltip.
If you click the button, you can input the note for the element.