This simple dialog displays the list of available text variables for insertion in your topic. The list includes both Help+Manual's predefined variables and any variables you have defined for your project. When you insert variables with this dialog they are highlighted as objects in the editor, like this:
This makes them easier to identify. This is only a convenience, however. You can also enter variables by typing them directly in the editor. When you publish your output Help+Manual will still identify them.
Selecting More... displays the Text Variables section in your Project Configuration so that you can edit existing variables or add new ones.