Contextual Help, displayed as a hanging tab under the toolbar, displays one or more Help items that are related to the Screen that is displayed in the work area.
Click on the Help tab to expand the Help available for a particular Screen, then click the Help hanging tab to collapse the Help display.
Not all features and functionality have Contextual Help, but many do, and can provide you with quick answers if you have questions about performing some task. Contextual Help is contextual, because the content found within these is specific to the task you are performing.
Contextual Help often consists of nested, tabbed content about the task you are performing. Click on the links on the left to navigate to different subjects. Within Contextual Help, you will often find links to more detail information, usually pointing to the WordPress Codex, which is the WordPress knowledgeable site.