There any many ways to get online help in Qt Designer. For example, you can read an online version of this manual by selecting Help/Manual from the menu or hitting Ctrl-F1. But the most interesting help feature will probably be the context help. When you edit a property in the property editor, you can just press the F1 key to be shown the part of the Qt reference documentation that explains exactly that property. Similarly, when a widget is selected in the form editor, pressing F1 will show the description of that widget in the Qt reference documentation. When the background of a form is selected, pressing F1 will get you the description of the Qt class that is the base class of the form class (usually QWidget, QWizard, or QDialog).
Whenever you are in the Qt help browser, which you open by pressing F1 or Shift-F1, you can access a comprehensive help index by hitting Shift-T or selecting Go/Topics/Index from the help browser menu.
Another way of getting help is the so-called What's This help. If you are uncertain what a particular toolbar button is for, for example, you can click on the toolbar button with the arrow and the question mark and then click on the toolbar button in question, and a small information window will pop up that explains what the button is for. All dialogs have What's This help as well; just hit the Shift-F1 key (you won't be able to use the toolbar button in a modal dialog) and click on the widget you are uncertain about. The same applies to all the views: the widget hierarchy view, the form list, and the property editor.