A frame is a user defined rectangular or sqaure region within the browser window that displays a Web page, alongside other pages in other frames. Here's an anology to get the drift. Think of a grid 3 boxes high and 3 boxes wide and in each box, a different web page. Your browser sees the entire grid of nine boxes as though it were on whole page.
PS Better answer, don't worry, frame ignorance is on your side if you decide to leave them out.