Context-sensitive help is used in software applications, both stand-alone applications and web applications. It is defined as help relevant to what the user is currently doing in your application. For example, if the user presses F1 or clicks on a Help button in a dialog, they would see a help topic describing that dialog – that is context-sensitive help.
Implementing context-sensitive help takes a little work up front but it will bring considerable benefits for you in the long run because it will make your product easier to understand for your users.
The options you have for implementing context-sensitive help depend on your output format, because different formats support different ways of making calls to your help files.
Using context-sensitive help
Context-Sensitive Help & Popups (Reference)
IDs, Context Numbers and Keywords (Reference)