Configuration Set
A Configuration Set is an Administration area component that allows configurations to be grouped in one place.
Without configuration sets, if you had 100 pages that all needed the same five configurations, you would have to manage 100 * 5 = 500 different configurations. If you had to change a page region assignment for the HTML configuration across all of these pages, this would have to be done 100 times; one for each page with that configuration.
Using configuration sets, you define the five configurations at the configuration set level, and then each page which uses that configuration set inherits the configuration and page region assignments.
Additional Notes
Note that configurations can be overridden at the page level for special circumstances (usually changing a block or stylesheet assignment). This does not affect the assignment of the page to the configuration set, and these changes can be "reverted", which removes the customization and defaults the configuration back to that of the configuration set.