Popup Help is the name for help messages that show up when the mouse hovers over a user interface element. Popup help provides context sensitive help and tips for how to use and understand the UI elements of TouchDesigner.
Popup Help on Expressions - when working with python expressions in parameters, popup help will appear for any partially selected part of the expression. For example, for the expression me.time.seconds*2, you can select-highlight the me.time.seconds part of the expression and the result of me.time.seconds will appear in a popup help.
Help message that comes up when rolling the cursor over the interface, or clicking the ? help button on the corner of each operator's Parameters page.