Popup Help is the name for Help messages that comes up when (1) moving the cursor over the TouchDesigner user interface, (2) clicking the ? help button on the corner of each operator's Parameters page and (3) holding the Alt key while moving the cursor over the parameter names in the Parameter Dialogs. Popup help provides context sensitive help and tips for the use 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.