CharmKeys is an extremely simple addon, designed to make it easy to mark targets in combat. It allows you to bind keys to assign a raid marker to the unit that is under the mouse cursor, whether it's a unit frame, name plate, or character in the game world. Since it uses the mouseover unit, you don't have to change targets in order to re-mark, vital for a tank or melee DPS.
The same effect can be accomplished with macros, which is why the addon is so extremely tiny. CharmKeys just allows you to save up to 9 macro slots by binding keys directly.