Game Rule Maps allow you to individually specify one or more game rules for a map of a certain game type. This means that you could, for example, switch off the Translocator for a particular map played in DeathMatch but leave it on for other game types that use that same map. Or you you could specifically set the Goal Score for a map.
The key to understanding Game Rule Maps is realising that they simply act as an "override" for the current game rules in play. So you don't have to specify all game rules for a map - you just specify the ones that you want to override.
On the top left of the screen you'll see a list game types available. The first thing to to is select the game type of the map you wish to alter.
On the top right of the screen you'll see a drop-down list of available maps for the game type that you selected. Simply select the map you wish to alter and click on Add.
On the bottom left of the screen is the list of game rules that you can choose to customise for the map. Select the appropriate game rule and click on the arrow button that points to the right (ie the add button). Only Add the game rules that you wish to override for this map and leave the others on the left list (ie not added).
On the bottom right of the screen is the list of game rules that you have customised for the map. To edit the game rule just select it and the control to edit it will appear below the list.