Click on the curson/question mark tool in the Standard toolbar to convert your cursor to the What's This? cursor.
Clicking on any item will open help documentation.
You may need to search or navigate to the appropriate item using the search function or table of contents.
Pressing Esc will get you out of the What's This? funcationality without clicking on anything.
It may be helpful to hover over the item in question when not in the What's This? functionality to obtain a name for the item that can searched for in the help documentation.