Frames consist of a number of html pages which display alongside each other. This allows one page to change while another remains constant - keeping a logotype, ad banner or navigation buttons in the same place and so easing navigation and reducing repetition of elements.
It is easier to change a constant element once in a frame than to track down multiple uses across a web site.
There is an html page for each visible frame, and in addition there is an enclosing frameset html document which keeps the sub pages/frames together.
In this simple layout with two visible frames - a yellow frame on the left to hold links/buttons, and a blue frame on the right which changes content dependent on which link is activated - there is actually a third containing frameset hidden from the viewer.
If you decide to use frames, this guide from the W3C should help