Context-sensitive Help is Help that is sensitive to the context from which it is called—that is, it “knows” what you're doing when you ask for Help and offers assistance on performing that specific task.
Before context-sensitive Help, integrating Help into an application was easy. The Help author wrote Help topics about an application, and when a user needed to access Help, she went to the Help menu, brought up the contents screen and navigated through the Help file until she found what she wanted.
For very simple applications, this model of Help authoring works well. Many users and programmers, however, prefer and demand a higher level of online Help. Help files, like the programs they document, have grown larger over the years. It is no longer convenient for a user to navigate through an entire Help file to find the section she needs. In a larger application, she may well never find it.
Enter context-sensitive Help. By reacting to the state of the application from which it is called, context-sensitive Help eliminates most of the navigation through the Help, searching for the relevant topics.