The essential utility of frames is the ability to control the contents of a given window by links outside that window. A moment ago, you clicked a link in the left frame (window) but the contents changed over here. How?
Each frame on a page has a name and a target. We use the name to specify the target. Here's a graphic that shows the names (banner, contents, and main) with the arrows showing the targets.
Here's the bit of HTML code (from 'contents') that produces this effect:
<base target="main">
Microsoft® FrontPage® knows to insert that code based on the setting in the Page Properties | Default Target Frame. Typically, you shouldn't need to change this setting, but if strange things start happening with your framesets, this is one of the first places to troubleshoot.
Note the two links in the top ('banner') frame. Clicking them changes the page within the frame named 'contents'. Also take note of the fact that the 'contents' frame does not have a target - absence a target means hyperlinks will act like any other non-framed page. Any links within 'contents' will be displayed in 'contents'.
To demonstrate, we'll put a link here to the 3rd and final page of this tutorial - More About Frames.