The screenshot above illustrates the tile selection (in red).
Edit a hex or paint the map: You can either paint the map (left click for 1 tile at once, right click for 7 tiles at once) or select a hex and edit it.
It was inspired by the built-in Civ4 in-game world builder. This mod provides an in-game editor for Civ5 to change the map on the fly or to serve as a cheating tool.