Frames are a way to divide the browser screen to allow easier navigation under some circumstances.
Frequently, frames are used to add a side menu bar to a website where the constant back and forth clicking would become tedious in a single page. In this example, the side menu bar would allow the user to just click in the side menu bar, and their choice would load into the main window. Your browser does not support frames. If you create a special page for those without frames, you may be doubling your work. It is best, with effort and practice, to create a page, in this case, main.htm , that will work in both frames and noframes browsers.