The src attribute indicates the file that should be used in the frame.
The value for the src attribute is a normal XHTML page, so you must have a corresponding page for each <frame /> element.
While the value of this attribute will generally be a file on your server, its value can be any URL, so you can use the src attribute to specify another site.
You might find that some search engines on the Internet (such as the image search on Google) will create a frameset whereby the top of the page remains the search site and the bottom of the page is the page you requested.
If you use a frame like this, it is good practice to offer a link that will close the top frame and allow the viewer to view just the content of the main frame (as Google does).