FRAMESET deals with large portions of a frames-based site. For instance, if you have a page with three frame cells, but you only want two of them to have blue borders on them, the other should have a red border. You would use this command to set aside those two frames and assign traits to just that section. There will be a specific subset of commands for use with SGML format styles that deals with this.
You can find more information on the FRAMESET command in the Frames Tutorial